Thread:Project:Support desk/Enabling CACHE ACCEL causes fatal exception of type mwexception/reply

Maybe your system doesn't have any PHP code acceleator listed in Manual:Cache.

The MediaWiki installer checks if a supported PHP accelerator is present. If it didn't detect it, enabling it on a running wiki may cause that error. Run the installer just to see if it detects the accelerator. Otherwise you should install it first.