An expired cert should be something that you can work around.
In Chrome, in the html area, (where roundcube should be) click on “Advanced”, then click on “proceed to {yourdomain.tld} (unsafe)”
And away you go.
You should be able to get a new, letsencrypt certificate by going to
https://box.yourdomain.tld/admin/
and clicking on system, choosing the TLS (SSL) certificaticates menu option and then click on replace certificate.
If those options aren’t available, it’s likely you are running an older version of MIAB, and I recommend you upgrade to the latest version here: