[_ck_]

The /~username is known to mess up both wordpress and bbpress permalinks. If you are trying to run that way permanently that way it will require some hacks.

You might be able to setup the username as subdomain if you know what you are doing and get around the problem ie.

username.example.com

Or there might still be some decent free subdomain services around, unsure – you could use one to park onto of your account and then run that way to solve your problem.