When composing a message in Roundcube, email address auto-complete does not work.
If you add at least one letter into the "To:" field and then leave it for multiple minutes you'll eventually see the following timeout popup appear for a few seconds before automatically closing:
This issue has been found to caused by incorrect IPv6 configuration on the server.
You can determine if your server has incorrectly configured IPv6 by first checking to see if IPv6 is enabled with the following command:
# sysctl -a | grep disable_ipv6
net.ipv6.conf.all.disable_ipv6 = 0
If you see that disable_ipv6 is set to 0, that means that IPv6 is enabled on your server.
If IPv6 is enabled try to ping an IPv6 domain:
# ping6 google.com
connect: Network is unreachable
If you get an error message instead of regular ping output, that means that your server's IPv6 configuration is not set up properly.
This can cause various problems, one being that Roundcube email address autocomplete fails to function properly.
You have 3 different options to resolve this issue:
1. Contact a network administrator and have them fix the IPv6 configuration of the server (Recommended)
2. Use this article to learn how to disable IPv6: How to Disable IPv6
3. Remove all AAAA records for all of your domains (Options 1 and 2 are much better solutions)