Overlay touching too many faces (max 64)

Issue has been solved, original post is quoted below for referencing to those who experience a similar issue

The error can occur if the “offending” brush is decently big and/or has small lightmap scale. Basically, try dividing the brush in two or more.

That’ll do 'er! Thanks HGrunt!

Splitting the ground up into 512x512 brush squares fixed it. - Solved.