Unknown condition metric '...' for Entity type 'device'

The alert page /alert_check/alert_test_id=<id>/ permanently displays the following error messages:

 
"Unknown condition metric 'weekday' for Entity type 'device'"
 
Screenshot:

Our configuration of the alert is:

device_status_type equals snmp

device_status equals 0

weekday le 6

time ge 0900

time le 1900

But with our settings set, alerting still works fine. Therefore, we simply assume that it could be a "display error". 

Our current Observium Version is Professional on revision 11008