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.
this error normally occurs when there are no users or there is no domain assigned to the user.
Adding a user with a domain should resolve the error.
to the admin user? Well in my case the admin indeed owns only a forwarding domain, but this is not the problem. Even is the admin does not have any domains, Softaculous works with the domains which belong to resellers.
Try this: create a forwarding domain at Plesk and call Softaculous as admin. You will see the list of all domains in a select box where you can choose the one you would like to manage. Select the forwarding domain and you'll get the error "ERROR IN SHIFTING TO USER LEVEL", and now to try to get back to the select box (the easiest way is to restart you browser to clear the cookie).
As your list is sorted by domain_id this error suddenly occurs when calling Softaculous if your domain with the lowest domain_id is a forwarding domain since this one is preselected.
A good advice would be to change the SQL statement so that Softaculous only lists hosting domains (and to order them by domain name alphabetically).