HTTP error 503
The service is unavailable.
Horde is unavailable while the system is being upgraded.
So sometimes Horde won’t work after a cpanel update. I found this post here:
https://forums.cpanel.net/threads/horde-wont-load-after-update-to-11-52.503071/
so through ssh run:
cd /var/cpanel/.application-locks
rm HordeUpdate\:username