I have check with out PG team at backend on this issue. Seems like there was an issue at backend due to which for certain period of time there was an issue for guest users to accept the invitation.
This issue has been fixed at backend
Hope for your as well the issue is resolved.
If you are still facing issue you can let us know so that we can take this further to our PG team.
Let me know if you have any further questions.
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.