When you try to perform MSSQL Database backup in DNN4Less Control Panel you receive the following error:

Error: Cannot open user default database. Login failed. Login failed for user 'xxxxxxxxxxxxx'.
Source: .Net SqlClient Data Provider

 Solution :

1. Login to the Control Panel: https://control.DNN4Less.com/cp
2. Select your Domain
3. Go to Websites & Domains 
4. Click Click Databases
5. Click Click Database Name
6. Check the database User Name 
7. Click Make Default for DB Webadmin