Can you test this:
Login with username (without @domain.tld) and try to set the identities.
I mean not a username like web1_user1 but the username as created on the user and e-mail tab in ISPConfig.
What is happening then?
If this works, it means that a login with only the username is better then to login with firstname.lastname@example.org
If this is true, i consider to change my howto at this point, which means i will not use the adapted main.inc.php for ispconfig anymore. In that situation only a login with the username will be possible.
Then saving identities will work. It's a compromise.
Let me know your experience!