I disabled one at a time. The 500 error for threads persisted with plugins disabled. I'm wondering how my host, at the time, was able to tell me there was a file error? Did they run a 'broken' url that resulted in the 500 error and saw what file was erroneous when called upon? Something similar? If so, how could I go about that and address that file issue? I don't know what caused it unfortunately since it was a month ago so I can't really narrow anything down.

For reference, here is a link. The URL seems off.. http://www.boostedk20.com/forums/sho...2800#post32800

What is your vBulletin version, and what is your php version?

^ Also has your host recently made any server-side changes or more importantly changes to mod_security or similar?

Edit: Or did you recently update the forum? This could be related to file permission/ownership if for example the files were uploaded via ftp under a different user account - that all depends on how the server is setup.

For this error, I think it caused by your htaccess file or your hosting.
Checking on that.

If you're getting internal server errors (500 status code), check the error logs of Apache or PHP and that should guide you in the right direction of what is going wrong.

vBulletin: 4.2.3 patch level 2; Current PHP version: 5.4

I added SSL about 5-6 weeks ago.
It may be, but they haven't been touched in a long time. Way before this issue occurred. I'd like to address that aspect, but I'm unsure it's causing this.
Do I check the error logs through vB or through my host/cpanel? I haven't checked logs before.

In your case, you can only check the errors logs through your host (whatever control panel they use).