M65 freezes during call transfer

configuration : M300 + 4x M65

firmware 530 007 both for base and M65

during an attempt to transfer an incoming (or outgoing) call to another M65, I can not finish the process, the phone hangs with a YES/NO dialog asking if I want to transfer the call. The two soft buttons below the yes and no are not responding, and the M65 is stuck. Only option is to hang up and this finishes the call obviously. I can hold, call internally another M65, switch back to the incoming call, even do a conference with incoming + 2 M65, but can not transfer the call. Tried almost everything including a hard reset and fresh start of the configuration. Any help please would be greatly appreciated !!