Security Token...
 

I'm running vBulletin 3.6.10 -

Every time I try to post anything, I get "Error: Security token was missing or mismatched"

No idea why the security token problem is coming up. Only product I installed was a usergroup legend that seemed to work fine, and that I've used in the past on previous vB powered forums. I can't find anything about this issue with any version except 3.7.0, which is odd...

Security tokens were introduced in 3.6.10, so it's not so odd that your searching is leading you to 3.7 versions. Here is a thread about this issue: Implementing CSRF Protection in modifications Did you recently upgrade to 3.6.10? If so, you need to make sure that take a look at any templates you chose not to revert and make sure you add the security token into any forms in those templates.

It's a fresh install ;)

You said you added a modification though. You need to make sure that any modifications are made for 3.6.10 or 3.7 or they will not have this security token present.

(A link to the forum and a test account may help.)