This is related to the different versions you are running:
Changes to the local database and sync in Drafts 52 make it incompatible with older versions. Once you install this version on any of your devices it will migrate your data and you must update all other devices you use Drafts on to 52.0 or greater to continue sync. Older version will temporarily disable sync, and warn you to update to continue sync.
If you still use Drafts on a device which cannot be updated to a newer OS version supported by Drafts, you can continue to use the old version of Drafts on that device, but sync will be disabled.
(Drafts 52 is out! Advanced Data Protection Support and more)