I do have a client1 that I added a longtime ago and everything works fine for it. The only thing I know that has changed since then is in the php.ini file I added some disable_functions but I have tried changing this back to no functions disabled at all and restarted apache and it makes no difference.
Thats the most likely reason for your problems. ISPConfig uses e.g. the exec function to add a user, if you disable thet, no users will get added anymore. There might be more then one php.ini file on your system, please check all these files and make sure that no functions are disabled.