I am having this issue as well. Have tried updating iOS on phone and watch, but still hanging. Only fix is to uninstall and reinstall. Works for one ride, then fails next time.
I found an issue on Support titled " Certain iOS Users Can’t Log In". It suggests that the hang has something to do with using the watch as a HR monitor and recommends not pairing the watch for HR until this is fixed. I have not tried this yet to see if it will work a second time.
This is pretty bad. I don’t feel like I should need to buy a separate HR monitor when I own an Apple watch. I don’ know if this is an Apple or Zwift issue, but please fix it soon. Doesn’t make sense to keep my paid subscription if this sort of thing continues.
Just my $0.02