UPDATES from the customer:
As you can see in attached screenshoots, old tape library with LTO5 drives has on drive setting Media Type=LTO-ULTRIUM and the list of available media pools includes "Default LTO-Ultrium" pool.
Otherwise, the new LTO-6 library has TAPE as Media Type and the list of Pools doesn't includes LTO-Ultrium.
Tape used for the restore is good, because the tape content import was successfull.