Hello,

After a certain time that I was busy on other thing I come back to fight this problem.

And I have resolved it.

How I resolved it ?

In first our server is a debian Buster.

I have checked the logs in "/var/log/odoo/odoo-prod-server.log" and I found this line :

2020-12-09 14:07:16,418 4452 WARNING odoo-prod odoo.addons.base.models.ir_actions_report: wkhtmltopdf: b"The switch --header-spacing, is not support using unpatched qt, and will be ignored.The switch --header-html, is not support using unpatched qt, and will be ignored.The switch --footer-html, is not support using unpatched qt, and will be ignored.QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-odoo'\n"

wkhtmltopdf:b"The switch --header-spacing, is not support using unpatched qt, and will be ignored...

Then I have stopped the odoo service, uninstalled the actual version of wkhtmltox (0.12.5) and replaced by the last version (0.12.6) with patched QT.

I have restarted the service and tested the pdf generation.

All is ok now.

Best regards