internal server error problem, how to check and debug?

Hi..

I have some problem
I added a https urls monitor ( Monitors > HTTP(s) URLs > )
it is a web application and can be accessed normally on my browser (local) and also from internet
and also run well on server where ApplicationsManager is installed (attahced applicationisok.png)

But when I add it to applications manager, It returns error (as attached in internalerror.png) :
- internal server error
- Data collection yet to start. Please refresh this page after the data collection interval has elapsed
- Health is critical as the resource is not available

when I add another url monitor, there are no error occured, applicationmanager can do poll and show the data.

Any suggestion on how to check those problem?

thanks before
Thomas