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 the security protocol. Horde is aware and working on the issue with no specific timeline.