The Horde webmail client can have issues communicating with the server. The error is something that can be fixed by using another email client like RoundCube. Also, it has been reported that clearing the browser's cache can fix the error.
Rebooting your computer and trying another browser may fix it as well. The issue has to do with a Horde cookie and browser cache conflicting with the security protocol. Horde is aware and working on the issue with no specific timeline.
Horde support is no longer available in cPanel and WHM starting with version 108. This is because Horde does not support PHP 8.x, and PHP 7.4 reached its End of Life in November 2022.
