From personal experience, you should disable all your mods if you have any (you should really use JSGME if you haven't already, it makes modding a hundred times easier).
After you've done that, navigate to "DCS World\bin\Run DCS Repair.exe" and run a repair.
After that, check if you can play multiplayer by joining random servers and seeing if it crashes, and if not, re enable your mods if you want to.
Please note, that texture mods and sound mods are really the only things that don't seem to affect weather you can join servers or not. Things like .lua and config edits will prevent you from joining from certain servers, or cause a CTD.
I hope this helps!