Check not working if host answers with "500 - Internal Server Error"

[mauthi]

Uptime monitor works fine with 1 exception:
One of my hosts repsonded with a 500 - Internal Server Error and the uptime monitor did not notify me.
I check afterwards the "uptime_last_check_date" and the date was not changed anymore since the site went down.

Is there any reason for that behavior or do I make something wrong?
Thx in advance for your help.