Issues with the devel module
Based on your comment about the 500 error and the menu entry problems, I suspect that the problem isn't really with the Devel module, but rather with your server.

Take a look at your Apache error_log to see what is causing the 500.  Following the steps for fixing a WSOD may also help.

A total guess is that you are running out of memory, and this is happening during the menu rebuild phase when you have an empty cache.  Enabling the Devel module likely was just the tipping point and now you are seeing memory problems.  You can try increasing the memory_limit and see if that fixes things.  But, you really do need to look at the error logs and find the true cause of the 500 errors.