Paul,
OK 1st things - from what I can see - you're the only user with an ATI SB - everyone else seems to have HDA Intel... so that's likely a different code path.
From what I can see - 4.19.x works for you, 5.3.x fails. Are you able to narrow down which build broke things for you? this will help identify code changes which caused the break.
Yup, the last good one was: uniaud32-linux-4.19.163-20201230c.zip, following that they were either a TRAP 000e, or the hard-lock. See the attached testing matrix.
For what it's worth, there were two other earlier releases that did actually show the same behaviour, they were:
1) uniaud32-linux-3.18.140-20200816.zip
2) uniaud32-linux-4.0.9-20200816.zip
To be honest, I'm not 100% it needs to be a null-model serial cable - I just capture the serial port in virtualbox and it works. To send log messages to serial, add /P:1 for com1 or /P:2 for com2
OK, may not be as hard then...let me see if I can just hook this thing up to one of my laptops, they are older machines so should have a serial port on one of them...
Thanks!