The base game works fine, however it seems like all of the large servers want me to have this file in order to play. However, I've got no idea just what this thing even is... help?
EDIT: Same for something called dcmod.pk3, due to missing these two files I seem to be unable to play on most servers.
Your trying to join dcmod servers, youll need dcmod. Its on jk3files.
I'm not sure if this will help but did you try cl_allowdownload 0 in your console?