Time Machine across network stopped working after 15.3.1 update

Time Machine across network stopped working after 15.3.1 update

MBPro 14" M1

Mac Mini M2

Mini has 8Tb external HD split into 3 partitions: 4Tb for data, 2Tb Mini TMbkup, 2Tb MBP TMbkup

Timemachine was working fine (sometimes slow over wifi for the MBP) for about 2 years.. until this weekend.

Both machines were on 15.2, did 15.3.1 update on both machines, now timemachine (and maybe finder?) is not seeing/connecting from the MBP to the Mini.

TMBackups working ok on the mini. TMBackups on the MBP failing "The backup disk is not available.."

Checked Sharing>FileSharing on the mini for the 3 partitions and should be ok, user(admin) and group R&W, everyone else Ronly. (later I've changed these to all be R&W and still same results)

Firewall is off, both machines.

Powercycled both machines several times (not just "restart" but "shutdown, wait, reboot" - heard somewhere does different things incl clearing cache), even unmounted ext HD, shutdown, unplug power to drive, wait, plug back in, reboot.

One thing I notice is on the MBP, in Finder, going to Network>machine>ConnectAs (disconnect and reconnect as the main/admin login), I can see the files ok on the account (mini's internal drive) but going to the data partition on the external doesnt show files, but does show a blue folder icon of 3 stickfigures holding hands - reason I checked the FileSharing on the mini at the start. Same with other partitions.

Anyone else have this issue? and solved it?

Thanks in advance.