Version 2021.2.3 is stable but there could be an issue if there was a change in version (or in the code) during recruitment which “broke” the study. Some people may have successfully done the study after it was broken if their computer was running an older version of the code (for example because they or someone else using their computer had already started the study before the break).
If you are using credits then you can run into issues if a lot of people start the study at the same time. The credits are reserved for 24 hours so you may need to assign more credits than you need in order to allow enough people to start.
The error you mention in point 2 might be related to multiple people starting within the same second. I think that it only affects the log file, but it might also affect the completion URL. Are you using database or CSV saving?