obscure error while displaying metadata when sp.key can not be read

[simevo]

at the end pf the docker-compose procedure when I visit the SP metadata endpoint http://localhost:8099/wp-login.php?sso=spid&metadata instead of the XML I get to see this:

This page contains the following errors:
error on line 2 at column 1: Extra content at the end of the document
Below is a rendering of the page up to the first error.

to reproduce, just make wp.key unaccessible to the docker_wp container with:

chmod 600 wp.key

to fix it, revert that:

chmod 644 wp.key

this is known to spid-php.lib as: italia/spid-php-lib#27