Acces is denied when trying to run templates in Template Builder

Question

I've installed Tridion 9.1 for a customer and almost everything seems to work except for one thing. When I try to run templates in Template Builder I receive the error:

"System.Web.Services.Protocols.SoapException: Access is denied"

in on the CME server I get

"Access is denied Component: CompoundTemplateWebService Errorcode: 0 User: NT AUTHORITY\NETWORK SERVICE"

Things I checked/tried already to resolve the issue:

1. Add Mtsuser to impersonation users.
2. Give MTS/networkservice in the policy editor access to start as batch job
3. Set Both users as Tridion user.

But until now no luck. Do any of you have pointers to check out? Help would be much appreciated.