Jupyter notebooks to A4 (again)

[Martin Schöön]

Hello all,

Some years ago I asked about exporting notebooks to pdf in
A4 rather than US Letter. I got help, rather detailed
instructions from you in general and Piet von Oostrum in
particular. Following the advice helped and I was happy.

Now it does not work any longer:

         nbconvert failed: A4article

I am stumped. I have not changed anything and all
looks OK.

Today I tried up-dating all things Python and Jupyter
but that did not help.

I have also tried removing the A4 stuff and after 
restarting Jupyter I can export to PDF and get US Letter
paper format.

A quick and (obviously) not very clever internet search
yielded nothing helpful.

Any ideas?

/Martin