I take it you left the car at home and went to work by other means?
If you've only had the car for a short time and already the rear brakes have seized on a couple of times, then you have a problem with them.
The only time my brakes have seized has been after launching the boat and left overnight with the hand brake on.
I believe I've seen reports on here from your neck of the woods where driving through deep snow then parking up with the hand brake overnight has seized the brakes - but they shouldn't seize just because its cold and moist.
My car developed a leak in 1 of the rear cylinders - so I decided to replace the whole rear brake system - cylinders, shoes, drums, springs, clips. I was expecting it to be an expensive job, but it wasn't, the parts are quite cheap and easy to install. It takes more time bleeding the brakes afterwards than fitting the bits. I was lucky in that the pipes leading to the cylinders were in good condition, in the UK these have often corroded and are damaged when disconnecting them from the cylinders (due to salt used on the roads) - don't know what its like in Finland - but they are expensive if you do not make them up yourself.
If you do replace the brakes and the cylinders are OK, you could just replace the shoes & drums so that you do not disturb those pipes. It should be said that people often put the brakes together wrong and they don't self adjust properly. If someone has replaced the brakes previous to you getting the car, they could have installed them incorrectly. Not sure if this would result in them binding like this.
I think I have seen a report on here where the rear brake shoes delaminated, and there have also been an alarming number of reports where the little clips that hold the shoes in place corrode, snap and the shoes fall off the cylinders leading to complete brake failure. I'm not sure if these would create the symptoms you describe.
As Nodge has (repeatedly
) said, it could be your IRD - you would definitely have left the car at home if it was. There are a couple of ways the IRD can fail to give these symptoms - eg gears driving the prop shaft PTO slipping or the splines taking drive from the gearbox stripping. If you have lost drive, then the splines have gone, if you still have drive, then they are intact. If you remove the prop shafts (only takes 20 to 30 mins but not nice under the car on a cold night) and the sound goes, then it is (more than likely) the IRD.
It could also just be the damper on the VCU has come loose and is rolling around on the prop shaft - this is quite a common occurrence. People usually just use an angle grinder to cut them in half and use the car without them. This would be the first thing to check - its easy!
These are just some thoughts. Sure there are other things it could be.