User on my website are sparadically getting a 403 Error in our forums. They are able to see other pages on the site but they get this error sometimes when they go to the forums. Any suggestions on what in the forum software could be causing this.

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Are you running the server? Or paying for shared hosting?
Because I would say this is definitely a server issue (not threads). So, an issue for your host to sort out.

The site is on it's own box.

I can only tell you that this happened to me a few years ago.
I contacted my host and it was fixed within the hour.

Sorry, I know this doesn't help much, but at least I can tell you that it's not a threads issue (or at least it wasn't in my case.)

BTW my site was and still is, on a shared server.

Anything odd in your webserver error logs. If you can repeat the issue, then check the error logs at that time to see if it has anything that might help.

The server company sent this to me and asked that I ask the software creators about this......

Looks like mod_security is blocking the requests with the following:

[Thu Jul 31 11:01:07 2008] [error] [client 38.99.161.194] ModSecurity: Warning. Match of "rx (?:\\\\b(?:(?:i(?:nterplay|hdr|d3)|m(?:ovi|thd)|(?:ex|jf)if|f(?:lv|ws)|varg|cws)\\\\b|r(?:iff\\\\b|ar!B)|gif)|B(?:%pdf|\\\\.ra)\\\\b)" against "RESPONSE_BODY" required. [id "970902"] [msg "PHP source code leakage"] [severity "WARNING"] [hostname "www.jamaicans.com"] [uri "/forums/ubbthreads.php"] [unique_id "@x9qzwoLCgIAAF@55R0AAABK"]
[Thu Jul 31 11:01:09 2008] [error] [client 72.12.29.207] ModSecurity: Warning. Match of "rx (?:\\\\b(?:(?:i(?:nterplay|hdr|d3)|m(?:ovi|thd)|(?:ex|jf)if|f(?:lv|ws)|varg|cws)\\\\b|r(?:iff\\\\b|ar!B)|gif)|B(?:%pdf|\\\\.ra)\\\\b)" against "RESPONSE_BODY" required. [id "970903"] [msg "ASP/JSP source code leakage"] [severity "WARNING"] [hostname "www.jamaicans.com"] [uri "/forums/ubbthreads.php?ubb=postlist&Board=8&page=1"] [unique_id "@0Ku6woLCgIAAF@55TkAAABt"]

ModSecurity rarely plays well with any forum software. It blocks way too much considering that the POST data in forums can contain just about anything.

Most people disable this completely for the domain or just for the forums directory.