unfortunatly app1.android has stopped

I create a new project cross-platform->pcl named App1. I make no change. I can compile all platforms. No problem. Then i start debug. All seems ok. The app seems to appear in the emulator but after a milliscond the error message "unfortunatly ... " appears and the app is gone. I tried it with genymotion, i tried it with visual studio emulator, the same result. i tried to uncheck "shared Runtime", no effect. I work with Win 10, vs2017. AMD Athlon X2, 64 Bit. On other PCs the same App works. Has anybody an idea? (HKLM WOW6432Node Android SDK Path is also correct )