I wouldn't rule out 'quirky' behaviour from any Leica digital M.
My camera is on FW 2.0.1.5 and I get lock-ups. Typical for me is while shooting, like mid sequence, suddenly the shutter 'half fires' as in, something happened but it didn't return to ready and nothing else functions. Mine might have a shutter problem, but I don't know for sure. There are other variations of lock-up too, such as how you described it. I think the closest I've come to a totally unresponsive camera (not even the shutter half firing) was while in Asia recently. I really don't know if temperature/humidity was the cause, but it was typically hot and humid - over 30C - and I was also using the EVF on and off. A few times, pressing buttons just wouldn't do anything.
I'd guess my shooting style is rather heavy compared to many others, in that I'll do a couple thousand images during the course of a day-long event. I'll usually get one or two lock-ups during this time. But I've also had similar situations to yours, in respect to the timing, where I'll have used the camera continuously for a while, have a break, resume shooting and within a frame or two, it locks up. I've had it lock-up within 5-10 frames at the start of the day. I can't figure out a pattern or what causes it. To be clear, lock-ups for me mean having to pull the battery. I think sometimes just cycling it off/on works, but lately I just immediately go to the battery.
You're right that the camera will reset some settings. From what I can determine, it resets to the parameters from when it was last turned off. Therefore, if you typically use a set configuration, you should set it and turn off the camera, then turn it on to use. For me the most annoying aspect of this is when the ISO setting the last time it was correctly turned off is not what I was using when it locked up. I have to remember to immediately set the correct ISO, and if time allows, cycle the camera off/on.
Of course I should send the camera to Leica, but I rely on it quite regularly and it's just such a pain to get serviced because it will take 6-8 weeks. I'll probably eventually do it near the end of the warranty period, but so far reliability of the camera doesn't seem to have deteriorated. Rather, it's a quirk I live with and try to work around. Oh, when this happens, you may lose images. Luckily for me so far none that really counted (for clients). If it locks up and the red write LED is flashing, try to wait it out as long as reasonably possible before popping the battery. It's possible the camera is still writing to the card, but extremely slowly (someone else recently posted about this), but no promises!
Oh, and try to avoid reviewing images while the camera is still writing to the card. It will lockout the shutter release in many cases until the buffer has fully cleared. At least my camera does.
There are a number of theories about why these lock-ups happen, including bad battery batches and camera sensitivity/incompatibility with some SD cards. You will read a lot of statements about which cards are best. IMO, there is no guarantee one type will work better than another.
I'd suggest you use the camera as much as possible in the return period to determine if it happens unacceptably often for your needs and likes.
