Time Machine over wifi fails with Error 30

We have a Time Machine partition on the 5N2 used by 4 Macs. Two are working great, back & restore over wifi are very fast; but recently the other two became ponderous, taking over 24 hours to complete a backup. After a week or two of that, Time Machine now fails on both with an Error 30:

The backup disk image “/Volumes/.timemachine/Drobo5N2.local/DED3617F-17E8-4ED0-BC95-F81709D749C6/DroboTimeMachine/MacBrian.backupbundle” could not be accessed (error 30).

Due to geography, ethernet would be very difficult, but we have a fast Orbi mesh network. Suggestions to get past the Error 30, and to prevent its recurrence?