Problems with Installation on Mac (M1) OSBigsur Log In doesn't appear after installation


I downloaded the desktop-client and it installed successfully, but no Log-In window pops up. When I click the icon, nothing happens - or - just a bit of wiggling…
Anyone experienced similar problems with newer Mac models?
I have a Mac Air (M1/2020) Big Sur 11.3.1. (solely bought for new job and next cloud sync) and I am a bit devastated.
I de-(installed) the app several times.
Many thanks in advance