Thanks ballscrewbob!
OSs are Windows 10 Version 1703 and 1607. I can install other boards via board manager -no hassle. So i ruled out compromised OSs or security software.
However, i had success using the JSON "description" for 2.0.2 version from "Releases · arduino/ArduinoCore-arc32 · GitHub". I have no idea of any differences between this version and the arduino IDE "built-in".
Fun thing: Again the CRC error occurs when using the JSON for 2.2.0 version from the same site.
For now it seems to work somehow, but i have to check this with the board now.
Genki