I had a few who has been meeting some challenges after upgrading to DPM 1801 regarding DPMDB protection.
When performing an upgrade of System Center Data Protection Manager, the old DPMDB is transformed into the 1801 configuration. What actually happens is that the upgrade process will add a “GUID look alike” sequence of characters after the DPMDB name.
The challenges that people has notified is that their DPMDB protection (in a DPM-DPM-DR perspective) has started to fail. The reason is that after the upgrade and the new name of the DPMDB the GUID will change and therefore the possibility to continue protecting the DPMDB.
To solve this you simple modify your protection group hosting the protection of the DPMDB and be very sure that you hit the refresh button at the bottom of the guide so you will see the DPMDB with the new name. Select the new DPMDB and click through the Wizard.