SOLVED!
My windows7 Task Manager does not list the processes anymore, as previous versions did, but you can still get the processes by:
open a command prompt
(run) tasklist
In there I saw that after closing Sonar (X3 Production), sonarpdr.exe is still listed as active process
Can be killed by running the following at the command prompt:
tskill sonarpdr
After that, I could start Sonar again, working as normal
So at least for this issue, Asio4All might cause this issue, but it can be overcome, and Sonar works now normally
Thanks for all your suggestions