Anyone who is getting an internal server error 500 when trying to activate or use plugins in WordPress 2.5: This is likely due to a memory issue in PHP. I’m not sure why, but I’ve noticed that the new version of WordPress seems to push much harder on the memory of a server, especially during plugin activation. It appears that this internal error 500 situation may be related to a PHP setting that by default limits PHP to using only 8MB per instance. The way to resolve this issue is to up the amount of memory usable by PHP to 16MB in the php.ini config file. In most cases you won’t be able to do this yourself but will need to request your server admin do this for you.