I have received an e-mail from QNAP Germany suppert, which is absolutely useless.
They said they cannot create the issue on their own and would recommend upgrading the FW again.
In addition I should reboot...
Hello, all of that I have done and wrote in my support request...I've done it numerous times by the way...no change...
My workaround by now is the following, and it might be working for others as well.
I have established SMS protocoll via bonjour as well, so in my Finder there is not only the NAS seen as NAS (AFP), but also NAS (Samba).
Even though I am not using the Samba connection I can now connect through AFP without getting the message every time. I still get it every tenth time or so and the overall mounting does take longer (but not 30 - 45 seconds as before).
As said it does work for me, though it is all but perfect or how it should be.