Thanks for your advice =)
Implementation: fcgid
i-mscp-Version: 1.3.0
Syslog throws the following:
Aug 21 16:43:17 v12941 proftpd[20849]: [myIP] (127.0.0.1[127.0.0.1]) - FTP session opened.
Aug 21 16:43:17 v12941 proftpd[20849]: [myIP] (127.0.0.1[127.0.0.1]) - [myFTPUser] chdir("/") failed: Keine Berechtigung
Aug 21 16:43:17 v12941 proftpd[20849]: [myIP] (127.0.0.1[127.0.0.1]) - FTP session closed.
Obviously permission issue. I'm trying to find out, how to solve it, at the moment.
Creating new ftp-users is possible again, but only with "/" as home directory. Login still doesn't work, too.
***EDIT****
I solved it!
I checked /etc/passwd and saw the following users:
i-mscp master web user with gid="1001" und uid="1001"
i-mscp web user with gid="1002" and uid="1002"
imscp database contains "admin" table with my domain-user that has "1001" in columns "admin_sys_gid" and "admin_sys_uid". I compared these fields with one from another new i-mscp installation on a separate test-host and noticed a difference.
I updated the values of both columns to "1002". Additionally I updated column "gid" in table "ftp_group" and columns "uid", "gid" in columns "ftp_users" for all entries.
After that, everything works fine again.
I hope, someone with the same problem can find this thread and saves a lot of time.