This post is to share my experience with Onprem ePO/CSR login page error and how it resolved. Couple of weeks back I noticed the below error on the login page and once login the similar kind of error noticed under the Menu - Extensions.
The issue is if you've an External SQL DB configured then the connectivity between ePO and SQL DB is crashed due to some reasons .
To resolve this issue you need an SQL authenticated account with the Public, db_owner and Sysadmin privileges.
Once you've the account enabled you need to verify the account under Menu - Report Server Settings - Database . Edit with the external DB and test . If the test is successful then logout and navigate to CSR server address/core/config and verify the DB connectivity is fine.
If both the above tests are fine , then restart the McAfee ePO Application server service along with the CSR server and later the error will be removed.