Have just spent the best part of and hour and a half on this, so I thought I'd share and avoid someone else the same waste of time.
ZCS 6.06p1 FOSS / Ubuntu8.04LTS 32-bit
Z-push 1.3RC3 BackendZimbra 32 / Ubuntu8.04LTS 32-bit
Self-signed SSL w/ Root cert installed on clients
Activesync failing with error 0x85010014 when syncing mail on WinMob6 client
All of the MS-centric info on the net talks about this being caused by SSL problems so I spent all of my time chasing that down. Turns out it was caused by an e-mail which couldn't be processed by the device. E-mail was image-only HTML mail with no non-HTML/non-CID content. Deleted the mail and the folder(s) started syncing again.
Hope it'll help someone else in the same position.