To me it doesn't sound like a faulty backup-battery. If it was the battery, you should only lose the stored settings if you pull the plug from the wall.
At least that's the way it is with my BM 5000 with a dead battery. I can store stations, volume settings etc. as long as i only turn it off with the remote (standby). But if I pull the plug -then everything is lost.

So - sadly - it seems that there is something more serious wrong with your receiver 🙁

I'd try rebooting it.  Unplug it and leave it unplugged for serval hours unit it cools down.  Then cross your fingers, plug it in, and hope all is well again.

Overnight unplugged: still no. 😮

Could be a processor fault but ripple on the supply lines can also cause "digital confusion".
Replace the capacitors on the processor board, the larger one for the 5V supply and also the 4-5 small ones between the regulators and mains transformer.
Check the rectifiers and check for cracked solder joints where connectors are soldered to the large amp/power board.
Still no luck, replace the processor and/or serial memory chip.

Martin

The smaller caps (including inside the CPU can) are actually already fresh, I happen to know 😕

The 4700 uF in the lower voltage supply is original, but as this happened suddenly, I'm afraid the thunder storm might have something to do with it. But after the rectifiers, worth checking some solderings as well as I seem to remember this particular Beomaster had some problems in that area.

This turned out to be a very peculiar case.

I sent the BM to a local B&O repairshop (small radio/tv-service which is the official repair-place for older B&O products). The serviceman called me few days ago and told that there was nothing wrong with the BM! We agreed that he will keep the BM for a couple of more days and if no problem is found, I get it back. 

Today I got it back. And it works. It just works.

Can't really complain, I didn't need to pay anything and the BM is back and working. I can only guess it was some bad soldering and it fixed itself on the way to the service. When the BM "broke" it had been switched on for several hours so maybe the heat triggered some bad soldering to open circuit.

I must say I am glad the BM is back!  In my current setup I listen only with headphones and my MacBook Pro's output didn't really do my Grado headphones much justice!

Happy end - but a strange story. 

I'm glad to hear that your Beomaster works again. And it's REALLY good service, that you didn't have to pay anything...Most repair shops charges you just to have a look.
I have a similar story of good service from around 10 years ago: My Beogram CDX had the typical problem with bad solderings -sometimes the sound disappeared and came back -some of the LED's were flashing, etc. I brought it to a B&O dealer near me called "Studio 24" where I used to buy a lot of my B&O stuff. They had the CD-player for a week -and the problem didn't show...I picked it up, and they told me to come back if the problem appeared again. I didn't have to pay anything.

I brought it home -and it worked like a dream for a month or so...then the problems started again. The sound disappeared, although the disc was still running and the counter was running.
I brought it back to the repair shop. After a few days i picked it up. They had fixed some bad solderings -and they told me that I didn't have to pay, because they should have found the fault the first time 🤒

Unfortunately the shop is closed now [:'(]

(edit) -and why isn't all of the smileys on this site working 😡😕

Yeah, it was good service, but yours, Steffen was even better by your description! The fact they admitted that they "should have found the fault in the first time" shows really high standard by the service center/repair man. 

I must say: listening to music is again fun! Simplicity and quality; I really suffered when the BM was away! Now, after just a little over 24h since getting the BM back I have already been listening to three FM stations, two CDs, two cassettes and other stuff thrue AUX. The daily-use remotes (Beo4 + MCP6500) are back right at hand and carefully dusted.

Having a little time without all this luxury, one really experiences how much these things really mean in daily life.

Unfortunately, the BM is going crazy again. Same symptoms as last time.

I opened it and remade some solderings but no difference. I don't think I have enough wits to continue from here; to the service center again...

🙁