host_error backward mins/maxs
"It means that the mins and maxs ( the bounding box ) for the sequence is "reversed" (one of the mins is bigger than the maxs). This happens when anything calls SET_SIZE with bad extents. If you compiling your own code, you can wrap the setsize() calls and check it yourself, but that's probably not a simple answer."
Translation: Do you have any special models or prefabs? they may have been screwed up, or you somehow turned a brush inside out, no idea how that could have happened - but some entities like platforms seem prone to this. You can also look for a brush with bad extents, or check the texture, is it divisible by 16 on the edges EXACTLY? Finally, are you using the most modern version of zoners tools? Old versions or ancient qtools are more prone to this error.