Okay. I just wanted to follow up with this thread for the sake of documentation; that way, in the future, other people having this problem know what I did to solve it.
I contacted my web host, and it turns out that it was an issue with ModSecurity.
This is how they explained it:
Quote:This issue can be caused ModSecurity, which is an Apache module that works as a web application firewall. It blocks known exploits and provides protection from a range of attacks against web applications.
Sometimes its rules can be triggered in a false positive by plugins or theme scripts. In your case, there were a few rules triggered, but not necessarily the same rules will be triggered for someone else.
Thus, we have to to test it on a case-by-case basis in order to determine which rules need to be whitelisted for each client. For your forums, we could do that once you tried an action and the logs told us exactly which rules were being triggered.
TL;DR > My web host needed to whitelist a few rules so that I and other users could, among other things, put apostrophes and quotation marks in our signatures, as well as put HTML in posts. Only they could whitelist these rules; I wouldn't have been able to do it without their assistance or permission.
So, to the future: check either ModSecurity or Suhosin, and be sure either to turn them off or to put exceptions in place.
Cheers.