Lost VE.Bus connection to one of my Multiplus II 5kW

Hello, I have a three phase system with 2 parallel Multiplus per phase (6 units at all). I do not know why but I lost one unit (the master of phase 1). When I switch it on mostly no LED is flashing and the converter do nothing. The others work correctly. When I try to connect the sytem to the configurator I find the 5 other converters, but not the failing one. I tried to connect the unit directly to the MK3, then I could find it, but only for some minutes, then it disapears again. It seems there are SW problems on this unit. What further tests ca be done or how could I reset the unit without an interface?

Hello,
to give information for the progress of solving this problem.
I found the failure. The VE.Bus connector RJ45 has a contact problem.
It was in a way that the VE Signal is routed through to all others Multis, but the Multi itself has not receive any VE signal. That was detected by the system and does not give the Multi the signal to start - therefore it is waiting and waitin and waiting.
Because of the connection problem, it is also not possible to reach it and find the problem or make an SW update.
I am a little bit disapointed that Victron use this unreliable RJ45 connectors for this very important signals that can stop the complete system.