I believe this has to do with " Signature check timed out "
I to have this issue. Which I believe it is also related to the "mission file not downloading" or getting stuck on the "receiving data" page.
I asked the server admin to turn off signature check and it allowed me in, though i could not use any vehicles and the AI/Enemy could not see me.
I believe too many people are having this issue/bug.
Please head over to https://feedback.bistudio.com/T120211 and check out my manifest and join in and ask for help/let them know you issue.
Ive changed the MTU, forwarded ports, DMZ'd, uninstalled reinstalled numerous times, made new profile. nothing works,, they need to fix this . please