Hi Jay
You may be onto something there. I came back to post that I resolved the problem by the following:

- I noticed that iCloud had renamed my Documents folder to "Documents - Dan's iMac", with "Documents" being a symlink to that. On a Macbook that didn't have the problem the folder was still named "Documents"

- I disabled iCloud syncing for Documents and Desktop and renamed the folder back to "Documents"

- When I ran BiaB the crash did not occur.

- I tried renaming Documents in various ways, but the problem still did not occur

- I re-enabled iCloud syncing and problem still doesn't occur

So there must have been some bit of temporary state that was causing the crash which was cleared up, either by disabling iCloud temporarily or by BiaB managing to do a successful save.

In any case it's working for me now. Never did hear back from PG support.

Dan