I got 3x SXTsq ac and set them up 1 as an PTP Bridge AP and other 2 as PTP Bridge CPE.
The problem is that only one client can connect at a time. The second client gets the "medium-access timeout" error. If I disconnect the 1st CPE then the second connects successfully and the 1st CPE will connect briefly and then fail with "medium-access timeout" . Everything is default/auto. Updating to v6.42 didn't help. There are no other APs in the lab. What am I doing wrong?

The licence you have is Level 3. You haven't the ability to setup an AP-Bridge with level 3 licence in the 1st SXTsq AC. You must have at least Level 4 licence in the equipment you want to use as AP.
The Level 3 licence is for PTP (Point to Point) links or for CPE (Client Premises Equipment) and not for PTMP (Point to Multi Point) like AP.
https://wiki.mikrotik.com/wiki/Manual:L ... nse_Levels

Wow, I would have never guessed it! Thanks!