checking your port - https://check-your-website.server-daten.de/?q=sip.eservizi.it%3A5061 - your installed certificate is revoked.
Revoked: The certificate is revoked.
And it sends the intermediate certificate that uses the
CN=ISRG Root X1, O=Internet Security Research Group, C=US
expires in 5336 days
That's good. But it's unknown if the error message happens because your certificate is revoked (so the error message may be not so good).
So: Hitting the limit -> wait, then create one new certificate, then install it.
And check, if the ISRG root is installed.
PS: The warning isn't so really relevant. It's more a client warning. If an older Windows client tries to connect your server, that may fail.
But your server doesn't send the root certificate (that's good), so it's not really a server problem.
It's a warning, not an error.