This started to happen on one machine, MacPro 10.14.6 XCode 11, the same project on a USB drive works on another MacPro (at the vacation cottage, so far away) same OS version and same XCode version. Even happens with a brand new project. I tried to remove all prefs deleted all derived data, still no luck, even with a new simple project. What more to remove? This is really annoying now.

OK, seems to have to do with signing (as Quinn mentioned). Has to set signing to Manual and Developer ID Application, then it worked to debug the application, with Hardened Runtime active. I know my account was renewed recently and it might have to do with that. The Apple Development cert is downloaded in XCode as "Untitled" and not with the hostname as key.