Plesk 12.x / Windows :: User Account Already Exists While Really Doesn't

I have a problem with one of my user accounts (let's call it testuser) in Plesk. System is Windows Server 2012 R2 with Plesk 12.In the Control Panel, the customer and subscription for testuser are removed, so there should not be any data left for the domain/user.

However, when I want to create a new subscription with the username testuser, Plesk gives the error "User account already exists". I tried to manually remove the domain and took the following steps:

Code:
C:Program Files (x86)ParallelsPleskbin>customer --remove testuser
An error occurred during removal of customer account: testuser does not exist

C:Program Files (x86)ParallelsPleskbin>webuser --remove testuser
The web user with the name 'testuser' does not exist

C:Program Files (x86)ParallelsPleskbin>ftpsubaccount --remove testuser -do
main testdomain.tld
Unable to remove FTP account: Unable to find service node for web service on dom
ain with id=0

C:Program Files (x86)ParallelsPleskbin>database --remove testuser
An error occurred during database removal: database does not exist

As described in this topic, I logged in to the MySQL server and tried to remove the user:

Code:

C:Program Files (x86)ParallelsPleskMySQLbin>mysql -P8306 -uadmin -p[admin-pass] psa
mysql> g SELECT id FROM `sys_users` WHERE login=`testuser`
ERROR 1054 (42S22): Unknown column 'testuser' in 'where clause'
mysql> g DELETE FROM `sys_users` WHERE login=`testuser`
ERROR 1054 (42S22): Unknown column 'testuser' in 'where clause'

Also, the inetpub/vhosts directory for the user is correctly removed and therefore doesn't exist. Are there any other steps I can take to remove the user manually and so be able to recreate the user?