[cups] Impossible to access to web interface

[Sal]

Hi,
after some upgrade now it's not possible anymore to access to the web
interface.

I'm using 2.3.1-12 but experience the same with 2.2.10-6+deb10u2 on
Debian.

When I access to localhost:631 I receive:

ERR_EMPTY_RESPONSE

while when I go to localhost:631/admin or localhost:631/help

I see a black page. In logs I can see that:

E [30/Apr/2020:18:37:54 +0000] [CGI] Unable to open template file
\"/usr/share/cups/templates/header.tmpl\" - Permission denied E
[30/Apr/2020:18:37:54 +0000] [CGI] Unable to open template file
\"/usr/share/cups/templates/help-header.tmpl\" - Permission denied E
[30/Apr/2020:18:37:54 +0000] [CGI] Unable to open template file
\"/usr/share/cups/templates/help-trailer.tmpl\" - Permission denied E
[30/Apr/2020:18:37:54 +0000] [CGI] Unable to open template file
\"/usr/share/cups/templates/trailer.tmpl\" - Permission denied

Any help?

Regards,
Sal