Yes it happened at the end. I now uninstalled the Arduino SAMD Boards and installed it back again with admin rights. Now that works.
I plugged my MKRZero on and tried to run the basic "blink" sketch, but while uploading the usb connection unplugs itself. Afterwards it plugs in again and no error message occurs. The sketch "is successfully uploaded" but the blink sketch did not run on the MKRZero. The console says:
"
Atmel SMART device 0x10010005 found
Device : ATSAMD21G18A
Chip ID : 10010005
Version : v2.0 [Arduino:XYZ] Dec 20 2016 15:36:46
Address : 8192
Pages : 3968
Page Size : 64 bytes
Total Size : 248KB
Planes : 1
Lock Regions : 16
Locked : none
Security : false
Boot Flash : true
BOD : true
BOR : true
Arduino : FAST_CHIP_ERASE
Arduino : FAST_MULTI_PAGE_WRITE
Arduino : CAN_CHECKSUM_MEMORY_BUFFER
Erase flash
done in 0.837 seconds
Write 10996 bytes to flash (172 pages)
[==============================] 100% (172/172 pages)
done in 0.088 seconds
Verify 10996 bytes of flash with checksum.
Verify successful
done in 0.011 seconds
CPU reset.
"
Can you help me on this one @in0 (or other users)?