I have RB260GS that worked for a year. In one day, the switch has dropped all its smart features and turned into dumb switch.
I tried restore firmware thru TFTP and got interesting message in RouterOS log The switch now have MAC address 00:00:00:00:01:01! Also, this action didn't help - switch is dumb as before.

How can it be possible and what I can do now?

How do you manage to run dhcp client on this hardware ?

It's recovery mode, as described here - https://wiki.mikrotik.com/wiki/SwOS#Rei ... S_firmware

Any news on this issue? I have a GSP doing the same thing. Customer unpacked it and it never really worked getting into the interface. Well, the switch responds but presents a logout button and says "loading" until it times out.

I have a GSP doing the same thing. Customer unpacked it and it never really worked getting into the interface. Well, the switch responds but presents a logout button and says "loading" until it times out.

Well, this actually sounds like a different issue (not what OP complained about). This is likely a browser cache issue- I've seen similar problems when trying to access RB260GSP using a browser that was previously used to access a RouterOS (not SwOS) device on the same default IP address (192.168.88.1). Try clearing the browser cache, or use another browser or another (clean) browser profile.

I have a handful of RB260GSs and GSPs that exhibit this exact problem. Clearing the browser cache or trying different browsers or hardware-resetting the switch doesn't help at all.

REALLY wish a MikroTik representative would chime in here and either acknowledge the problem or give us a proven-to-work fix.