Try switching to the SMF default theme instead of the custom one.
Profile -> look and layout -> current theme -> change -> SMF Default theme (curve)
If the problem persists, it may be SMF. If the problem doesn't persist, it's the custom themes here.
Normally I'd say if the problem persists it IS SMF, but given that there are flaws in the output even on the default theme like:
<!-- Bad Behavior 2.2.15 run time: 0.000 ms -->
<a href="http://bg.battletech.com/forums/honeypot/mirandaattraction.php"><span style="display:none !important;">Register</span></a>
being inside <head> where the <a> and <span> tags aren't just invalid, technically they don't even EXIST... (I'm a little surprised given how SMF implements gzip compression it's not breaking there?!?)
Can't say I've heard of "mirandaattraction" but given what it is doing to the markup? One can only assume whoever made that doesn't know enough about HTML to be making such tools. Those ALONE could be causing MASSIVE compatibility issues; hence the 7 validation errors reported in the default theme and 40+ errors of the custom one. Either that or it has been added to the forum incorrectly like Google Analytics was. This is particularly true since having content/body tags in <head> will cause the browser to prematurely close the <head> and start the <body> DOM (what the validator refers to as "assuming <object>") making any LINK or other <head> specific tags that follow also be invalid or fail to process properly.
Though at least it looks like they pulled the Google Analytics code from it's invalid location between </head><body> -- since that should have been broken in pretty much everything OTHER than IE8/earlier and FF. (If you REALLY want that pointless broken bandwidth waster, it should go right before </body> in the template_footer() function in index.template.php)
The latest builds of SMF really "grinds my gears" with some of the garbage front end code -- like the massive amount of static scripting in the markup, loading scripts in <head> where they are slower, inline execution instead of DOM modification...
But as I said someplace else, what's really sad is SMF is STILL the best of a bad lot. Compare to the garbage phpBB, vBull or Xenforo vomits up and has the giant pair of donkey brass to call 'markup', and the difference is night and day. I might rag on SMF for wasting 56k to do 15k's job code-wise, but at least it's not the 150-200k that Xenforo developers seem to have deluded themselves into thinking they "need".
Still, so many things that are laughably bad even in the default skin... like just the heading and user bar are such... gah:
<div id="wrapper" style="width: 90%">
<div id="header"><div class="frame">
<div id="top_section">
<h1 class="forumtitle">
<a href="http://bg.battletech.com/forums/index.php">BattleTech - The Board Game of Armored Combat</a>
</h1>
<img id="upshrink" src="http://bg.battletech.com/forums/Themes/default/images/upshrink.png" alt="*" title="Shrink or expand the header." style="display: none;" />
<img id="smflogo" src="http://bg.battletech.com/forums/Themes/default/images/smflogo.png" alt="Simple Machines Forum" title="Simple Machines Forum" />
</div>
<div id="upper_section" class="middletext">
<div class="user">
<p class="avatar"><img src="http://d15yciz5bluc83.cloudfront.net/forums/user-avatars/avatar_1_1440161473.png" alt="" class="avatar" /></p>
<ul class="reset">
<li class="greeting">Hello <span>deathshadow</span></li>
<li><a href="http://bg.battletech.com/forums/unread/">Show unread posts since last visit.</a></li>
<li><a href="http://bg.battletech.com/forums/unreadreplies/">Show new replies to your posts.</a></li>
<li>28 August 2015, 15:47:22</li>
</ul>
</div>
Static style in the markup, at least three unnecessary DIV, absolute URI's for no good reason apart from wasting bandwidth, elements in the markup that are scripting only (so don't belong in the markup), what the blue blazes makes a user's avatar a grammatical paragraph, much less how is the greeting, two links and the date a "unordered list"? It's extremely unlikely that needs to be much more than:
div id="top">
<h1>
<a href="/forums/index.php">
BattleTech - The Board Game of Armored Combat
</a>
</h1>
<div class="shrinker" data-target="header" data-text="the header"></div>
<div id="header">
<div id="userInfo">
<img
src="http://d15yciz5bluc83.cloudfront.net/forums/user-avatars/avatar_1_1440161473.png"
alt="Deathshadow's Avatar"
>
<div>
Hello <span>deathshadow</span><br>
<a href="/forums/unread/">Show unread posts since last visit.</a><br>
<a href="/forums/unreadreplies/">Show new replies to your posts.</a><br>
28 August 2015, 15:47:22
</div>
<!-- #userInfo -->
Apart from outdated thinking, broken methodology, and a failure on the part of SMF's developers (since again I'm talking the default skin) to grasp the simplest of concepts like inheritance or separation. That takes 1156 bytes of code and drops it to 643 bytes -- pretty much tossing 45% of it away and would laughably be MORE functional. Anything NOt there would either be in the CSS or the Scripting, both of which can be CACHED resulting in lower bandwidth use and faster page loads.
... and that's the default skin from SMF. Good luck to anyone trying to take that and make it sane or base custom skins off it -- yet sadly again SMF is actually BETTER than most of what else is out there.
Of course as I discovered when I was working on it, FIXING the markup can create it's own host of problems as it can break certain mods -- admittedly why I used kid gloves on modding the forum and on the whole am against the general idea of forum and CMS mods/plugins/extensions. You can see why by just looking at the CVS for things like vBull or Wordpress where 95%+ of their vulnerabilities the past five years were not in the core software, but were created by popular mods/plugins/extensions/whateverTheyreCallingThemThisWeek.
Just like what happened with phpBB v2 where the core software was so crippled you had to mod it to the gills for the most basic of functionality (like avatars and attachments) -- said mods often meant people were unwilling to upgrade rising the modifications breaking, letting the "neverNoSanity" aka "Santy Worm" take down two thirds of the web.
Honestly after that, I'm shocked anyone still runs phpBB.
Now that my health is kind-of returning (thanks to taking 16 pills a day now along with enough insulin to kill an elephant) I should probably dust off my SMF skin replacement and finish it (Was at like 60% completion) -- only problem it really had was so many mods require the busted-*** broken-down bad markup as hooks to apply themselves, it tended to break such things.
But that's a struggle a lot of "established" software often has to deal with. Wordpress sees it all the time where gaping vulnerabilities and security holes remain unpatched because fixing them would break a lot of existing websites functionality -- in fact fixing some of it's biggest security flubs (like putting the username, hostname and PW for SQL into DEFINE, the ridiculous number of entry vector points -- might as well paint a glow in the dark bullseye on it for crackers) would pretty much break half the system's functionality, particularly on backwards compatibility with skins and mods. Things got a bit better with version 3/newer, but most of the problems still exist as it's got a "one wall" security policy, instead of a proper layered approach. SO much wishful thinking going on inside it that it's outright scary anyone would trust it -- but they can't actually fix it as it grew in popularity (for christmas knows what reason) faster than the maintainers could keep up with it.
SMF and other forum softwares face similar issues, the classic "do we fix this and break everything, do a half-assed job and sweep it under the rug, or leave it be and hope for the best" -- sadly these days the latter two are the choices most developers are making.
In that way forum and CMS software as time passes starts to become more and more like Windows -- the balancing act between "what we must fix" and "what fixing it will break". Or OSX with it's "Security through obscurity" where people basically are coasting on borrowed time, what with twenty year old BSD kernel bugs that remain unpatched in Darwin, the BSD/machs kernel mashup on which Apple built it. Apple's lucking that most crackers seem to assume that Apple users are already suffering enough so they don't bother targeting them -- particularly when at most cracking competitions like Pwn2own the Apple devices are historically the first to fall.
So many flaws, so much ignorance, so many people doing things not qualified to be doing them, and so little interest in doing things properly anymore. "Get it out the door, now now now, who cares how badly it hurts us in the long term!" -- It's an industry-wide epidemic. See how Windows 10 is as big a case of Microsoft flipping the double-bird at accessibility users as Windows 8 was to the desktop.
*SIGH*