I'm running a years old version of The Dark Mod / Dark Radiant, so maybe "Bad Internal Format" isn't even a thing anymore, but right now if I type "r_showprimitives 1" in game, I get lots of "WARNING:R_BitsForInternalFormat: bad internalFormat:34842", bloating up my qconsole.log file.
How do I achieve a GOOD "Internal Format"?
It started appearing right around the time when I made about nine visportals in my level, but although I could start deleting those and see if it helps, I really like visportals. They are my friends, and I can't imagine mapping without them.