If the target server is using a self-signed SSL certificate, how does this affect the PRTG monitoring and what must be done?

PRTG, by default, will reject self-signed certificates as untrusted, resulting in connection timeouts or errors. To monitor a server using a self-signed certificate, you need to configure the PRTG sensor to trust the certificate. This involves importing the self-signed certificate into the PRTG probe server's trusted root certificate store (or within PRTG's settings, depending on the sensor type). Be aware of the security implications of trusting self-signed certificates.