The waiting to join error seems to crop up for various reasons depending upon perhaps user system configuration.
I am not sure anyone has come up with definative answer to the question.
Whatever the reason, it is something that will continue to plague MP users as one cannot expect many users to resort to system configurations or call upon other technical considerations and expertise. The software was never designed to be primarily used by computer system configuration experts and users should not be expected to be one.
I was once told by a member of TGC that It should work by and large the same on everyones machine and so it should but it does not.
MP has numerous issues which make it impossible to seriously consider for practical use by the average user in my own personal experience.
Hopefully any future update may see some improvement.