[DaveBCC]

I have what could be considered the opposite setup (PC TB3 <-> apple TB3-TB2 adapter <-> 5m optical TB2 cable <-> Antelope Orion 32+ gen 3), and it has worked like a champ for over 3 years now, never had an issue.

For a long time there were issues with the optical TB2 cable transmitters (the lasers) burning up due to heat dumping from TB2 sources (in your case, the Mac Pro TB2 card). It might be that your TB2 cable has gotten damaged.