Ok, I found the solution.
We use a tool to reflash some information on the ARECA controller as we sell it in a branded version. Our production dep. adds our company name as "vendor name" in the ARECA firmware and the company name containes an "ü". Seems like the ESXI does not like that.
After reflashing and exchanging the "ü" with "ue", rebuilding the RAID and a restart, everything was fine.
Just if someone is interested.
I am having probably the same issue but I did not understand your answer.
Can you explain a little bit more please. Where did you change the ü by ue ??
Thanks in advance