At one of our Plesk installations I had the problem that the admin user always got the error "ERROR IN SHIFTING TO USER LEVEL" when trying to open Softaculous. After a lot of debugging and code inspection I finally found the error: the domain with the lowest domain_id in the database (in my case this was id 1) was not of a hosting domain but of type forwarding. As the following ones are all of type hosting my workaround was simply to delete the domain and create it again so that it got a new domain_id. Maybe it would be a good advise to fix the SQL query in the responsible script.