Well... I created this account because I had troubles, but meanwhile found the solution already.
But perhaps somebody else may have the same problem, so I want to describe it.
I used mysqldump and saved my data base, then I upgraded my server to Debian Wheezy and installed new versions of all software.
I restored the database from the dump file, installed Roundcube - and there was nothing. No contacts, no identities.
I logged into my server, logged into mysql, looked into the Roundcube database - and there was everything!
I used Roundcube and created a new identity. I looked again into the Roundcube database, and there was the new identity!
And then I found the reason: there is a column "user_id" which was 2 for the new identity and 1 for the rest from the backup.
The rest was easy:
update identities set user_id = 2;
update contacts set user_id = 2;
and everything was shown in the Roundcube web menu! ;D
If you look in the users table you can see what changed between the accounts that caused the problem.