That can happen sometimes if the server load is high while you're trying to log in to roundcube. Since Johnny is an experimental server the load is frequently high. If you want a faster server with less load and less errors we recommend switching to Ricky or Tommy. Which email address is exhibiting this behavior so I can try resetting the roundcube database for you?
That link no longer works because we added https to the deletion script, but that old website doesn't support https. Try this link instead
https://www.heliohost.org/scripts/delete.php