Roundcube time outs instead of accessing user email account

Symptoms

Roundcube timeouts occur instead of displaying the user's inbox, and errors similar to the following can be seen in the user's Roundcube error log.

[07-May-2021 11:44:01 Africa/Cairo] Failed connect to 203.0.113.2:2080; Connection timed out
[09-May-2021 12:10:30 Africa/Cairo] Failed connect to 203.0.113.2:2080; Connection timed out
[23-May-2021 09:33:27 Africa/Cairo] Failed connect to 203.0.113.2:2080; Connection timed out

Description 

The most common reason for this is that inbound TCP port 2080, which is used by the cpdavd service, is blocked. 

Workaround

Ensure that inbound TCP port 2080 is allowed through your firewall.

Additional resources

How to use nmap to troubleshoot port connectivity issues