XF 1.5 Forbidden Errors

[Gossamer]

I'm currently running XF 1.5.8 and my members have been coming across Forbidden Errors over the past week. The error says "You don't have permission to access ..... on this server". They pop up only sometimes when they are attempting to save a post. Only some users appear to be impacted so far. Also, when it pops up, the member who triggered it often gets their IP automatically blocked by our host.

I've been going back and forth with my host on the issue, but they haven't been able to fix it yet and have mostly just been whitelisting IPs as they get blocked. Does anybody know what might be causing this error so I could give them some more helpful information?

This is the sort of thing they're seeing:

 

[Paul B]

That is a web server error, typically caused by mod security.

Your host will need to identify and whitelist the rule which is being triggered.
Or disable mod security.