I have install the last release package from ispconfig.org.
The roundcube don't save the preference and only close the session.
I read all the instructions.
I've had the same problem but only wanted to change the identity for the sender's address to be for ex. email@example.com
instead of having firstname.lastname@example.org
and the name Leszek instead of web2_leszek.
A workaround to this was changing the line 670 within /program/include/main.inc file from:
670 $line = trim(str_replace("www.", "", $line));
and loging in by the username email@example.com
instead of firstname.lastname@example.org.Then
my identity looked like it should.
The rest of the preferences could be easilly saved.
I've fixed it on another server by using MySQL as the db.