Cause :
Error: Cannot open user default database. Login failed. Login failed for user 'xxxxxxxxxxxxx'.
Source: .Net SqlClient Data Provider
Solution :
Open Plesk > Click Websites & Domains > Click Databases > Click Database Name > Click Database Username and change follow things:
Change database user name xxxxxxxxxxxx to xxxxxxxxxxxx1
Old password xxxxxxxxxxx
New password xxxxxxxxxxx
Confirm password xxxxxxxxxxx
That's it.