Same issue here on FW2.1.8, HW 3.0 – sporadic reconnect failure each couple of days.
Connected to AVM Fritz!Box 7590 via FRITZ!Repeater 1750E.
DHCP enabled (Although Router configured to always assign same address)
Tried FW upgrade to 2.1.8(4) today, after reading this thread I will also try static address setting.
Last resort would be daily powercycle via manual timer on wall outlet, which I want to avoid, because it would need to withstand the current draw of my pump and switches.