RoundCube times out on NAT servers due to domain.tld:2080 being unreachable.

Symptoms

On NAT servers running Calendar and Contacts Server (CCS), Roundcube will not load, and entries similar to the following may be found in the user's carddav_http.log log file.

cURL error 7: Failed to connect to webmail.domain.tld port 2080: Connection timed out (see https://curl.haxx.se/libcurl/c/libcurl-errors.html) for https://webmail.domain.tld:2080

Description 

When CCS is installed, Roundcube attempt to contact the CCS service on port 2080 on the server's public IP address. If the NAT loopback is not configured or misconfigured, the server cannot contact itself on port 2080. Roundcube's connection to the CCS service times out, which prevents Roundcube from loading.

Workaround

Your system or network administrator must correct the NAT loopback configuration so the server can contact itself through its public IP address on port 2080.

Please note that NAT loopback (hairpin NAT) is required on any cPanel server with NAT.

Additional resources

What NAT configuration does cPanel support?