Time Machine stopped backing up
MacBook Air 13" mid-2011 Mac OS 10.13.6
Have previously been backing up successfully to a Drobo FS connected via Wi-Fi to an AirPort Extreme, but backups have started failing with no other diagnostics. Created a new Time Machine share on the Drobo, and TM recognizes it and finds it, then goes through the 'Preparing Backup' stage, starts the backup, hangs around for several hours and then reports 'Backup failed'. My desktop Mac (mini 2019, 10.14.6) backs up successfully to another TM share on the same device, although via hard-wired Ethernet to the AirPort Extreme.
All ideas appreciated...!
MacBook Air