We're running version 3.0.2 with PHP Version 5.3.28 on a forum that has around 50 posts per day. The code has been unaltered for nearly 2 months but a day ago it suddenly started exhibiting strange behaviour. The code has been verified against a local copy that it is unaltered and no additional files have been added to the system.

After a user has logged in and posted a message, they are unable to view the message, even if they use a different browser. Other forum users can see the posting. If another user now logs in the forum initially welcomes them with their name, but if they then go to a thread to post they are now told they have the identity of the previous user. This has also happened with forum moderators, allowing their options to be used by ordinary forum users.

We have moved the entire source code and database to another web host and tested that it works normally there. That host was running PHP Version 5.3.2-1ubuntu4.21.

Has anyone seen this behaviour or any ideas what may be the cause?

We have the answer now. Not sure yet what was changed by the web hosting, but it was simply that the server was caching pages rather than sending refreshed versions. This stopped it showing the new posts to the user who made them. It also made it send out the last version of a page that another user had seen. Solution was to modify the .htaccess to force all pages to be refreshed. In case anyone need that, here's the addition that worked for us:

<FilesMatch "\.(html|htm|php)$">
FileETag None
<ifModule mod_headers.c>
Header unset ETag
Header set Cache-Control "max-age=0, no-cache, no-store, must-revalidate"
Header set Pragma "no-cache"
Header set Expires "Wed, 11 Jan 1984 05:00:00 GMT"
</ifModule>
</FilesMatch>

Glad you have found the solution.

Yes, from what you described, it definitely seems to be a caching issue, but why your hosting provider has this, is an "enigma".

Whatever I could recommend is to swap the hosting to whatever more reliable.

Good luck!