As @andreas-doehler said. To fix the WATO plugin, go to ~/local/share/check_mk/web/plugins/wato and edit the file rocketchat-notification.py. Locate the following statement near the bottom of the file:
default_value = "http://" + socket.gethostname() + "/" + (
defaults.omd_site and defaults.omd_site + "/" or "") + "check_mk/",
Replace defaults.omd_site with config.omd_site() so that it reads
default_value = "http://" + socket.gethostname() + "/" + (
config.omd_site() and config.omd_site() + "/" or "") + "check_mk/",
You may also try
default_value = local_site_url,
local_site_url is a function defined in ~/lib/python/cmk/gui/plugins/wato/notifications.py. I’m not sure if it is available from within the local plugins.
This should be possible at the head of the file there is then an import statement needed. from cmk.gui.plugins.wato.notifications import local_site_url