Solved by granting permissions using powershell
First find out what webapplication the database mentioned in the event belongs to, then grant permissions. In the example below I’m using a test account but in production it were application pool accounts for our portals that lacked the permissions to mysites DB.
PS C:\Windows\system32> $w = Get-SPWebApplication http://portal.test:8088
PS C:\Windows\system32> $w.GrantAccessToProcessIdentity(“TEST\user”)
Event ID: 3760
Task Category: Database
User: NT AUTHORITY\IUSR
SQL Database ‘SP_Cont_MySites’ on SQL Server instance ‘sp_core_db’ not found. Additional error information from SQL Server is included below.
Cannot open database “SP_Cont_MySites” requested by the login. The login failed.
Login failed for user ‘TEST\user’.