It's kind of tough to say for sure. My initial thought was possibly some type of mailer issue, but mails are sent out after the post is made, so the post should at least go through properly.
Since it's just giving a generic 500 error, then it's not giving much info to work from.
My second guess is that it could be a mod_security issue (meaning maybe your host has recently enabled mod_security) but that normally would generate a different error. But just to be safe I'd check with your host and see if they have enabled mod_security.
If they say no, then you could check to see if it is giving any more information in the webserver error logs. This might give a bit more info as to what's going wrong.
If it's not giving any info at all then there are a couple options.
Option A would be an advanced support ticket for this to diagnose and fix.
Option B would be if you purchase an upgrade install. We'd upgrade your forums to the latest version and fix the issue if it's still an issue after the upgrade.
