Since latest version 3.0 Nagstamon checks the validity of the monitor server’s certificate as default. This check can be disabled, but many users will see an error message in their Nagstamon status window when first starting:

Pressing the ‘Fix error’-button will show up the server’s settings dialog.

Here the problem can be kind of semi-solved by simply ignoring it:

Way more secure is giving Nagstamon the certificate of the used certification authority. Please make sure it comes in Base-64 encoded PEM format:

As result of these settings Nagstamon will tell you as before what’s wrong: