There are more of those.
I have two APN, one was of type "default,supl" for internet, and one was of type "mms" . It seems the problem is fixed by setting the first APN type to "default", by removing ",supl" from the type.
Why the APN type should lead gps to trigger wifi disconnects I've no idea. It haven't checked if the wifi really disconnected at the link kernel layer or if it only delivers a disconnect event at the dalvik layer, but it certainly was enough to make openvpn disconnect (in turn losing networking until it would connect again).