Tech support has confirmed that the file I sent them does what I describe below but while I am waiting for their solution I wonder if anyone else has had this problem.
It only started with 2017 and I am using all the latest patches.

When a song has at least 5 RT's I sometimes find that BIAB locks up when I freeze the song and hit play.
Any song with more than 5 RT's is even worse and nearly always locks up when frozen.
It NEVER happens if the song is not frozen and I can toggle between lockup/no lockup by freezing/unfreezing.
It happens on two different computers with ample resources even if I enable the "slow machine" options for RT's
I am running as administrator and have reset to factory defaults.

Perhaps someone could try to re-produce this problem - try with a song with 7 RT's to make it more likely.

Thanks
Tony