Thank you very much! This worked.
I removed both read caches and then I could create the new disk group. After that I created the read caches again without a problem.
I have just one more question. The dgB01 disk-group is showing "46% VRSC" now for some reason. What does this mean?
I googled and found something about scrubing, which sounds kind of like it's eraising the entire disk group.. But maybe it's some automatic "health check" thing?