This is the server side copy routine in b2 saying it can't copy between buckets.
They are using the fallback path with download and upload.
However b2 recently upgraded the API so that you can copy between buckets, so if you try the latest beta you should find that it does server side copy between buckets.
I did the code for this, but forgot to merge it!
If you try the latest beta in 30 mins you should find it supports server side copy between buckets.