After day long experimenting to solve the problem, I started with the paid addons. The Milviz helpdesk suggested that a reinstallment of the simulator might help. I did so and after a very long installation and much to my surprise everythig worked normal again. The only difference I noticed was the location of the Community folder. It was now a bit up in the tree and was found in ... Appdata/Local/MSFSPackages/ Community. Moving the addons in was no problem. Mandatory Update 7 was not shown in the Program manager and I do not know if it was included in the update. Anyway the sim works perfectly again.
Truly excellent news, and I hope it helps others, too.