Thanks for the library management hints.
I deleted the old library and re-imported everything and
checked thatthe path to file location source parameter string is correct.
However the bug repeats like earlier.
Then i reinstalled 15.04 64 and installed jriver to internal HD1
and imported all "recordings" from internal HD2, All parameter values looked ok,
Then restarts for OS and JRiver started and the old bug repeats with same message (picture below).
(but the backup hd alternative is not any more visible due to complete fresh installation).
But I found an interesting clumsy workaround with which the error message does not come.
First the error comes and the I just start ubuntu "files manager"
and click inside the file manager to second hd2 and click into some folders,
So just clicking and browsing - no other actions.
Then I jump back to MC21 window and re-try the play the recording and it works and no error message.
any ideas what to check next to get rid of the clumsy workaround ?
(here other topic about how to fix broken links
http://yabb.jriver.com/interact/index.php?topic=50504.0 which did not help in my case)