ERR Failed to get response from the unit 1.

Hi We have 2 Dell N1548P Firmware 6.6.0.13 running. 

Switch does what he is supposed to do but no Connection via ssh, https or Console is possible. (Telenet and plain-http disabled)

I conected the console port to the second Unit of the Stack an trying to connect to Master Unit geting the following error:

<187> Nov 30 15:34:14 0.0.0.0-2 RMT_SESS[emWeb]: rmt_session_api.c(84) 29633746 %% ERR Failed to get response from the unit 1.

Will try to restart the Stack but need to wait until no one is on Campus. Did anyone run into something like this?