NEDAP PR50SB-BS SolarRouter power inverter problem

[Mikaelin]

Hello,

I have a 5KW solar router power inverter NEDAP PR50SB-BS that works only under low loads. For example, in a normal sunny day, I can watch TV powered by this power router with no problem. The power from the solar pannels converted to AC by this power router is feeding my TV (local out) and the over production power is fed in the public grid line. But if I try to start a microwave owen or 1KW heater, the power router is clicking (the relay is switching house power from solar to the grid, that is normal) but the problem is - it never switches back to local out power when I unplug all energy consumers, it stays every time grid connected. It is trying to switch to local out power without succes in a loop, endless clicking. The only way to make it work again from solar is to perform a reboot from the built-in control panel.
I don't understand why it behaves like that. Nothing changed in the configuration. Before all was OK, if the sum of consumed power is greater than the solar production power, then the router is taking current from the grid and switching back to the solar power when the consumed energy is less than solar production power (this is normal operation).
On the front  panel there is a LED error that never triggers. So is no error reported. When it tries to switch back to solar energy, the LED Operational is steady ON and the Grid LED is blincking.
The power router is without battery and was configured by a certified technician a few years ago and all worked fine in this mode. I was happy.
I can't contact the manufacturer because NEDAP is closed since 2016 and the 5 years warranty is expired.
Please help with an advice if you have an idea what the problem could be.
The link to the manual is here.
Thank you