Backblaze B2 stopped working at version V57.
The reason is that the Backblaze b2sdk library had been updated to version 1.16 which is no longer compatible with Python 3.6 as installed on MIAB. The solution is to revert to the latest version of b2sdk which works with python 3.6
As help to anyone else facing this issue, the solution I used was to download the previous b2sdk library using pip:
pip install -Iv b2sdk==1.14.1
and then copy the b2sdk from the location where it is installed to the location where MIAB wants the files, e.g.
sudo cp /installdirectory/dist-packages/b2sdk/ /usr/local/lib/mailinabox/env/lib/python3.6/site-packages/ -R
after that reboot the box and Backblaze backups should work again.