Hey @holger80!
Wow, you found an issue I was not even aware could happen!
Thanks for reporting, the error is raised on steem-python level, so for fixing it locally on the project, I would have to "down-port" the signature functionality.
I'll take a look into submitting a PR on steem-python itself, and I can walk you the lines of code that lead to this error, if you want to submit a bug report on steem-python itself.
(But pretty sure you know this code way better than me :) )
Thanks!