In fact the problem seem to be here since ubuntu 18.04. (my mistake in 1st post)
But everybody don't care because of WIFI.
But if you don't want wifi, it's a very bad news.
Thanks anyway.
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Exactly same problem with epson sx400 on freebox server
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https://bugs.launchpad.net/bugs/1895852
Title:
can't access network printer since 20.04
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Status changed to 'Confirmed' because the bug affects multiple users.
** Changed in: system-config-printer (Ubuntu)
Status: New => Confirmed
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https://bugs.launchpad.net/bugs/189585
Same issue on my computes + NAS, with Ubuntu 20.04.
I applied the same settings that in the case of 16.04.
Result: 20.04 - nothing is printed. 16.04 - print without any question or issue.
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Here the output of system-config-printer --debug with 16.04 (working version).
It can found the printer without problem or asking password.
So I think this problem is a regression.
** Attachment added: "Adding network printer with 16.04: not problem"
https://bugs.launchpad.net/ubuntu/+source/