XF 1.1 Error 403 - Forbidden

All forums and all threads? Or just some?

403 is a generic server error. It can be caused by permissions or restrictions like mod_security. I suspect mod_security. The only way to know for sure is to examine the logs on the server. Ask your host to disable mod_security to see if that is the problem.
 
+1 for mod_security, I've seen that being the cause of that issue before. You could try adding a disable statement in your .htaccess
Code:
<IfModule mod_security.c>
  SecFilterEngine Off
  SecFilterScanPOST Off
</IfModule>
 
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