Reseller can't create new user

[LI-Marcel]

One of our resellers is having a problem trying to create a new user. The error he gets is:

Cannot create user using those settings

Details

You cannot create more Users than you are allowed domains.
You currently have 25 Users and a maximum of 25 domains.

This is despite the control panel showing the following:

Used Max
Disk Space (MB) 3108.8 6144.0
Bandwidth (GB) 3.1890 60.000
E-Mails 38 500
Ftp Accounts 14 unlimited
Databases 16 25
Domains 13 25
Users 12

Does anyone have any ideas why this is happening please?

Regards,
Marcel