Since a few days, my emails are impossible do reach from IMAP, Nextcloud is down, and the web home page is also down, but the domain name is still correct and I can connect to the server with SSH.
Instead of trying to investigate, as it was an old installation on Ubuntu 14.04 I installed a new server on Ubuntu bionic.
During the restoration of the backup on the new server, it ended with the message
Error '[Errno 17] File exists' processing ssl/ssl_certificate.pem
And now, I have exactly the same symptoms as with my old install.
I removed and installed certificates 2 time (why it didn’t works the first time)
rm -rf /home/user-data/ssl/* && sudo mailinabox
And everything is working fine again.