Delete it cant actually , you will be able to see that in your database all forum data is preserved as it is .. try a "Forget your Pass" ? it might mail it to you .. ?
That's a good suggestion. The system should be able to mail/reset his password - what bothers me is that 99.9% of the time if you can't login after an upgrade, it has to do with the membership settings. I don't know if his database dates back prior to 1.9.1 or if it started as 1.9.3 -- In the first posts, it was an upgrade from 1.9.1 to 1.9.3 and so that's why I made note that the membership had to be changed. Then, later in the help, it was an upgrade from 1.9.3 to 1.9.5 - so there's a real up-in-the-air issue here.
Because the forum is in a language that barely translates for me, it's hard for me to test and diagnose.
He wasn't getting a ASP.NET error -- but whenever you try to login, it accepts the username and password, then dies with the yaf internal error "An error has occurred" and redirects to the forum root. With customErrors turned off, it still does this, and doesn't throw an exception - so it's an error that yaf is catching and logging.
There's a chance browsing the database for the YAF event errors might reveal something, but I'm not sure.
If you can't find it using the forum search, try my signature link -- searches this site using Google: Google is my Friend