Hello,
thanks for the reply. It seems to me the same way, but as far as the flash-client downloaded the plugin but didn't deploy it, I wanted someone to confirm if there is no chance to use in both clients. Since I am still stuck with vCenter 6.5 due to some software dependencies and the html5-client is not feature-complete in this version, it's a pity to be forced to alternate between both clients, depending on what task you need to perform. But I understand this, it is not effective to maintain the old frontend for guys like me 
Well, I'm considering this thread as resolved, anyway. Thanks for your time.
David