Hi Pert,
thanks for you post. I did the try to launch the app from the Terminal. Oh, I was close to say nothing happens and then after 2 Minutes I got this Message in the Terminal:
/Applications/Arduino.app/Contents/MacOS/Arduino
2017-11-23 12:40:41.142 Arduino[10697:974485] IMKInputSession presentFunctionRowItemTextInputViewWithEndpoint:completionHandler: : [self textInputContext]=0x0 *NO* NSRemoteViewController to client, NSError=Error Domain=NSCocoaErrorDomain Code=4099 "The connection from pid 0 was invalidated from this process." UserInfo={NSDebugDescription=The connection from pid 0 was invalidated from this process.}, com.apple.inputmethod.EmojiFunctionRowItem
But thats all at the moment. I'll leave the Terminal open for a while. It may happens something else laster 
So far. Anyone more Ideas? Should I link the Topic to github als well?
Cheers, Chris