For those who haven't seen the other issue, this has been resolved, at least in this case:
This may be an issue with storage of sign-in credentials, added in 3.0; every sign in credential gets added to the Windows Credential Manager, and if it ends up storing too many of these credentials, the sign in can fail to open.
If this is the case, then it can be solved by deleting all of the Survey123Connect entries within Windows Credential Manager, which can be accessed through the Control Panel.