This problem can be somewhat complex as it can result from different issues. You're on the right path starting with checking the Group Policy configuration and testing it. Based on the error code (0x80070bc4), there could be a problem with the printer driver or there could be a permissions issue. Here are a few things you can check out:
1. **Printer Drivers:** Make sure the printer drivers are installed correctly on the server and they were not recently updated. If they have been updated or changed, consider rolling back to the previous version to see if that fixes the issue. You can also try updating the drivers if they haven't been updated recently.
2. **Permissions:** Check to ensure that the users have the necessary permissions to be able to use the printers. Sometimes a configuration change can cause permissions to be changed or reset.
3. **Print Server:** Take a look at the event logs on the print server to see if any errors are being thrown there. This might give you an deeper insight into what's happening.
4. **Group Policy Update:** Try to manually force a group policy update on the user's machine by using the command "gpupdate /force" in cmd. After running this command, restart the PC to see if the changes take effect.
5. **GPO settings:** Look at the GPO that is causing issues, and check if the printer mapping settings are correct. These settings may have accidentally been changed.
6. **Alternative GPOs:** There could also be another GPO that is interfering with these printers. Look for any recently modified or created GPOs that might conflict with your printer mapping GPO.
Please provide updates on your progress and additional information so I can give you more detailed instructions or troubleshoot further if needed.