I was certain that as soon as I posted I would find a solution, perhaps not a solution but the problem ceased.
I still don't know what it was, but the corrupt teamwork file was over 100 Mb. When I saved it as a solo project I noticed it was only 48 Mb. After working with it for a week I again created a new TW file.
It remains 48 Mb and does not crash when sending and most importantly is creating backups!