I had the same issue which popped up suddenly, but probably after a firmware and driver upgrade on the VeeAm host server. Tape inventories, format operations and backups all failed after a random number of the library's tape changes. Firmware update of library and LTO didn't help.
With inventory operations I noticed that VeeAm mounted a tape and while it was inventorying, tried to mount another tape, and another, and another, which all failed.
We use SAS cabling, and in our case there are two cables (merged into one port) going from library to the host's controller. Windows' device manager showed two instances of our MSL2024 library and two instances of the library's LTO5 drive (we have only one MSL2024 changer with one LTO5 drive fitted). After disabling one of each in Device Manager, everything worked again.
It's strange, because we also have a Microsoft Data Protection Manager server with the same equipment. The Device Manager on that server also shows the devices in duplicate, but DPM has never had any trouble with it. Nor does the HP tape utility, that can diagnose the library's robotics. It seems a VeeAm thing. Don't know about version 9 though, we have version 8 (2084).