**problem is solved. I had to renew the permissions described like here (LINK). **
Thank you all for tryinig to help!!!
I bought a new Mac and installed a totally new system, not from a backup. When Stack Updates are shown, I can install them, but it seems they are saved in a wrong place. When I restart RW, the same stacks are shown again in the update-window.
I use the feature to select a place for the add-on folder, the stacks are all there in a folder named “stacks”.
Do you have any idea to solve this problem?
Thank you for your help!