there's a sticky topic on Compiling errors, this is what it says:
ERROR: FloatPlane: bad normal
Somehow you've created an invalid brush in your map. At least one face of this brush will be invisible in game. This usually is the result of careless vertex-editing.
SOLUTION: Use the Rogue Plugin to remove bad brushes. (.ace file requires Winrar)
Create a folder named "plugins" in the directory where your mohradiant files are (eg. Mohtools/plugins). Extract the plugins to that new folder and you're set to go. Start Mohradiant and in the menu look for the "Plugins" option next to "LOD terrain".
yeah that's what I was thinking when I mentioned the axes-lines in radiant. The compiler always starts by slicing the map along the x- and y-axis. If a brush is flush with one of those axes it could produce errors, but it's unlikely.
I've had the weirdest errors while building maps and it sometimes didn't make any sense at all. Best thing to do is delete and rebuild that room and try different things, like thickening that wall a bit, or rebuilding the room a little different. Just improvise.