GZIP HTML Output
If your PHP installation has been compiled with the ZLIB libraries, then you can enable this option to compress outgoing pages and save bandwidth.
The client's browser must support GZIP for this to work. If the browser does not support GZIP, then the uncompressed page will be sent instead.
If you are using mod_gzip on your server, then you are already compressing outgoing pages and there is no need to enable this option.
It doesn't make sense to me that if you turn this off then your forum gets broken. Did you turn it back on to 'fix' your forum or are you stuck at the moment with no access?