I'm curious as to when this issue started happening and any changes you've made to your game, ie. adding a bunch of addons or changing a network setting, that may have caused this. Are you being prompted with anything when you try to join the server, or just crashing?
In the meantime, attempt the good old Verifying Cache Files and trying again.
Also, go into your console (prior to joining) and type the following
+clientport 27005
press enter
Finally you can always try joining the map you'll be playing on but in a SP game prior to join.
I'm going to assume neither of those work, otherwise I wouldn't be asking questions to try and further diagnose the issue.