I'll answer myself:
here is the complete procedure, which also works for the C400.
Hi, I can't update the firmware on my C400 from 67.31.81 to the bridge version.
(When I try updating via the web server, an error occurs, same when I try to upload the firmware manually. When I use the update option on the printer, I get an 026-724 error. Nothing works.)
I've sort of given up on the networked options and when I looked for USB options, I only found the one provided by michel46 here (https://forum.support.xerox.com/t5/Printing/Firmware-upgrade-failed/m-p/250677#M27192). It's actually for a different printer but I do get the menu. Problem is, at the end of a rather too short period it's still the old firmware on the machine.
If someone could confirm that this is the right method, or which one is, that would be much appreciated. Any other suggestions much appreciated, too.