Cannot add new user
 

I'm running ISPmanager 5 Lite on Debian 7

An attempt to add a new user failed with the following error: "An error occurred while working with this user. User '' does not exist"

How can I solve this?

Logs are attached below

Feb 10 23:45:41 [4012:90] proc EXTINFO Run '/bin/sh -c useradd\ -M\ -o\ -u500\ -d\ /var/www/test/data\ -g500\ -p\ \"\$PASS\"\ -s\ /bin/date\ -c\ \"\"\ test' pid 4370

It seems that uid 500 or group 500 is busy. Please check passwd and group files.