[sfx1999]

Here is the solution (click me)

brush with coplanar faces

Example:
Entity 10, Brush 0, Side 5: has a coplanar plane at (-753, -9, 251), texture CA1X_CON1B
Entity 10, Brush 0, Side 6: has a coplanar plane at (-753, -32, 251), texture CA1X_CON1B
This is always caused by vertex manipulation.
Say you have a five sided object like the following diagram:


Moving the top point down to make the object into a square, will cause this error, as now 2 faces are on the same plane, which is not allowed:


To fix this problem, either move the vertex causing the coplanar warning to make the brush convex again, or move the rogue vertex onto one of its neighbors to destroy it.