Kudos to KOG AUDIO and T+A for their great service. Thank you very much!
Reply as follows in case it may be of interest to anybody else.
Apple has chosen not to accept older drivers in their latest OS versions any more.
For the older TENOR USB receiver (as used in MP3000HV and DAC8) there will be no new drivers released,
because the manufacturer of this chip went out of business. So the problems with playing audio files from a Mac are caused by these new OS versions. Up to the older OS X Mavericks (10.9) it must function.
The chips of this manufacturer were used by a lot of companies (like Accuphase, Marantz, TEAC, Lindemann, and many other).
All these manufacturers are facing the same problem with the new versions of MAC OS.
The key to solving this issue lies with Apple.
They only have to accept the old (and flawlessly working) drivers in their current OS and everything is ok.
I hope, these informations are helpful.