Sounds like something changed on your host.
Uploading your old cache files will only upload static info.
Also deleting the /forum/cache_builders/ and custom folder was a boo boo.
You should have only deleted in the /forum/cache folder.
You need to restore them. They create the files in the cache folder. Without them they will never get rebuilt.
After you put them back in:
rebuild cache from the control panel
Then:
I would start with looking at the cache folder to see if the files were rebuilt.
Also I would check that file permissions are correct for same as well as cache builders and all the other folders.
Maybe the host updated your site and something is hosed now.
