The manual is your friend:
"Only A6-style BSP levels and non-cached terrain are supported by the AABB collision system."
http://manual.conitec.net/aenable_polycollision.htm So you need to set your terrain to nonchunked. This is no bug report, so I'm moving it out of the bug forum. Anyway, in my experience you should leave enable_polycollision at its default value. A7 mesh levels and chunked terrain are much faster than A6 levels and non chunked terrain, at least in my project.