I can no longer generate PDF or send email …
A red single page with only this text appear :
An Error Was Encountered
Unable to load the requested class: Zugferdxml
Thanks for your help.
Please disable Zugferd for PDFs in the system settings.
Reference: https://development.invoiceplane.com/browse/IP-529
This is really not a solution, just a workaround.
It worked for me. 