All Minecoins gone & Achievement Rewards can't be collected...

I had like 839 Minecoins and now they are all just GONE! I suddenly just have 0 Minecoins.
The following happened on Win10:
I just wanted to collect my Achievement Rerward for "Oooh shiny" (the watch). This also didn't work. In other words: It was stuck in the purchase screen. I closed the game and reopened it but out of nowhere all my Minecoins were gone!! The watch was still not collected.

I restarted the game to check if the Coins really were gone and yes, still 0...

Then I checked on my Android 10 device (same account): 0 Minecoins.

Could you please explain me what happened and get me my coins back! Also I want to make sure that I can claim the Reward without all Coins disappearing again...

Tank you

Jonas