Today I lowered the printbed to the lowest point - with manual control - from a halfway height position. It broke the coupler. I think it wouldn’t be broken if lowered from the home position because the maximum height is in the settings.
Is there anything to do to prevent such an accident?
The only way to prevent this (as you said) is to home the unit before lowering it
I did almost the same with my nozzles.
I finished a print (1mm hight) and recalibrated the nozzles, and then I did the HOME ALL.
Yes, I had forgotten to set the Z to ist start point, so the bed came up and was hitting both nozzles.
Luckily, I have springs for bed adjustment, so the bed was pushed down due to the springs 
Nothing died