Deleting MPMissionsCache.

How to delete MPMissionsCache folder.

Easy steps.
Copy and paste or type this out yourself.

Press Windows+R and type CMD
Press Okay and then type the below into the Command Prompt.

cd %appdata%\..\Local\Arma 3\
rmdir MPMissionsCache /S

At this point press Y and then enter.

Then type below.

mkdir MPMissionsCache

Now you can close or type exit in the Command Prompt and reload ARMA.
Try reconnecting to the server you were on.