Originally Posted by Hans
Have a look at your database for ISPConfig.
In phpMyAdmin, fist select the ISPConfigdatabase (default= ispconfigdb) then go to isp_isp_user table and check your user records.
If you find the user which must be deleted, delete it by pressing the red cross.
Warning: be carefull do not screw your database!
At the other hand, if you can't find the user within the database it does not excists there.
I do not recommend deleting records manually. If you delete only the record in the isp_isp_user table, you destry the referential integrity of the database, as the permission record in isp_nodes and the dependency record in isp_dep is not deleted.