Z-axis moving up while printing??

[SebastianM]

Hey guys,
unfortunately, I have the next problem with my A10...

2 hours ago, I started to test-print the a10_dog.gcode sample. It looked good, but when I left the room for 5 minutes and came back, I saw something weird... the nozzle had moved about 2 cm up and it was printing in the air.. I never had something like this with any of my other printers...
It looks like it just printed for some hours without filament, but thats not the case... we are talking about 5minutes...
any suggestions O_O?

You can see it here:
https://youtu.be/jI-Kq7QO8bo