It can't load or download maps.
I have the maps, but it won't load them. Just stops at the loading screen listing this message:
Requesting maps/ub_rooftop.bsp
Failed to open maps/ub_rooftop.tmp
Requesting sounds/ub_sounds/traffic.wav
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ERROR: CL_ParseServerMessage: Illegible server message (0)
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I can load the games solo, though.