SharePoint 2013 Cannot complete this action. Please try again

"Cannot complete this action. Please try again", one of the scary error in SharePoint.

I was doing the CU update on SharePoint with SharePoint 2013 on Premise, after successful CU update, all sites home page getting this weird "Cannot complete this action" error screen.

CU update itself took around 8-9 hours for all 11 servers. Lots of lots of debugging and reading the ULS logs didn't help me with only one option left to rollback to previous version by restoring the Database and Servers.

Here are some of the issues we faced:

• Home page was showing above error.
• ULS log was having error some times "cobalt.exception" error.
• When we tried to access some of the Page Layouts files we got "The URL is invalid. It may refer to a nonexistent file or folder or refer to a valid file that is not in the current Web" error.
• When we tried to create any new page or list we got "There are characters in the page URL name that are not valid. Type a different name" error.

We created a sandbox environment and replicated the production environment, after doing lot of comparison with other environment and analysis we found the issue was with the Database Permission changed after CU update.

The app pool account used for web application was not the DB_owner on the Content Databases after CU update. where as prior to update the same Content DB the app pool account was DB_Owner. After reading Microsoft document I found below article which was talks about "SP_DATA_Access" role.

ref: https://docs.microsoft.com/en-us/SharePoint/install/account-permissions-and-security-settings-in-sharepoint-2013#sp_data_access-database-role

Make sure you try the following:

1. Verify "SP_Data_Access" has right kind of permission as mentioned in above link
   In SharePoint 2013....
   
   The Add-SPShellAdmin cmdlet does not grant the user membership  to the db_owner role, but instead places the user in a SPDataAccess role
   
   The SP_DATA_ACCESS role replaces the db_owner role in SharePoint 2013
2. if above doesn't work then give DB_Owner role for app pool account on individual database.
3. if step 2 works then "SP_Data_Access" role doesn't have proper permission.
4. if step 2 fails then give app pool account sysadmin role. (This will resolve the above error definitely).

Updated on 09/20/2018: 
     Hi Guys, we found out the actual reason for above error in error log. i am sorry its very late to update this.
     We are using Metalogix RBS. There was access denied on one of the Metalogix Stored Procedure. unfortunately it wasn't logged as "Unexpected" or "Error" that the reason we could not found this in Log since we were searching for Error.

Root Cause:

• Whenever we enabled Metalogix Job for Externalization on any Database Metalogix was adding some extra rbs roles and DB_Owner Permission.
• After our CU Installation, DB_Owner permission was removed by Microsoft since SPDATAAccess was already present on all databases.
• Unfortunately, our App Pool was not able to access the Metalogix functions and stored Procedure hence we received the error.
• after adding the DB_Owner permission to the database where RBS is enabled by Metalogix after that the error is gone.

Moral of the Story:

• If you get this king of error make sure app pool has proper permission on DBs or SharePoint Objects. 

Hope this helps!