Front end users - occassional problem

[scotch33]

This is a problem that happens to a few members in the feusers section of a site I run.

There are two member groups - pending and members

Certain members are unable to login - instead recieving this message...

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, webmaster@ile.org.uk and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

It appears that by deleting the member, and then reinstalling them that all works fine.

Anyone any ideas why this should be?