could be a cipher issue - try it (for debugging purposes) with
cipher none
You can also consider playing with
no-replay
although I wouldn't expect it to help.
If you change the MTU setting in openvpn on windows then remember to also decrease the MTU size of the tap-win32 adapter - openvpn cannot do this automatically on all versions of windows. You can change the tap-win32 mtu size using either 'netsh' or by adjusting the advanced TCP/IP properties of the tap-win32 adapter.
