Apple Remote Desktop - connecting to 11.5 BigSur, 'viewer and server are incompatible'

Having an issue connecting either via Screen Sharing app or Apple Remote Desktop. Upon connection establishment, the window size changes very rapidly in succession until eventually the app settles with "Authentication failed to xxx because the server and viewer are incompatible'.

ARDP 3.9, remote machine as tested either 11.4 or 11.5 (same mac, just upgraded - no noticeable difference).

I can connect to other remote machines with the same versions with no issues,

Thanks!

Apple Remote Desktop 3.9 is 4 years old.
Big Sur supposedly requires ARD 3.9.4, according to the App Store.

However my main Mac is stuck on Mojave, so I only have 3.9.3, which is perfectly capable of connecting to an M1 iMac on macOS 11.4 (& after your reminder, now tested on 11.5.1 ;) running client version 3.9.8 [as well as older clients back to El Capitan running 3.9.3.]