unknown error 1815 (0x717)

I must admit I have never seen this error before. Also, there might be few things on what it means, but I could not find on MS site at what point this error is triggered (but I'm sure it should be somewhere there).

Can you tell us what Windows version your are running? (and what patch level)

My impression is that you will get this error if either something really bad is wrong with the code (I would expect other then to have the same error) or something used by program (library aka dll) is corrupted or/and deleted. Last one is easiest to fix - just try to reinstall whole software package. If some library used by NW (and belonging to system) is bad then this might be difficult to see (do you see any other extra message next to above one - anything in event logs?). At least this is how I would start.

Can you also tell us when do you get this error?
- during labeling
- during backup (write)
- during restore (read)
- when end of tape is reached
- etc...

Doing:

net helpmsg 1815

I get the error back:

The specified resource language ID cannot be found in the image file.

So the problem appears to relate somewhat to the languages installed on your system. No idea what though!

As it is reading data from the tape I would guess you are doing a restore - could you possibly be recovering data to a system which does not have the relevant language resources installed?