@schnetz, @Rebekka
I tested what now works with 2.4.0p17. Yes, input validation for MonitoredHost/MonitoredService now works. Unfortunately, you cannot use it because you will lose all configured MonitoredHost/MonitoredService data if you run into any input data violation. The other option would be use no input validation at all in rules with MonitoredHost/MonitoredService form specs. As this might lead to inproper configured rules, this is also no good option.
Meaning without fixing
the first fix is of no use (to me) 
but thanks for looking into it
Thomas