App reports the server as untrusted (insecure certificates)

Observation

The ProCall mobile app for Android or iOS reports the own server with an SSL certificate as insecure.
The browser itself does not report any abnormalities.

Background

Browsers are more tolerant when it comes to the quality of certificates. 
The verification must therefore be carried out via other procedures.

Solution

Verify the quality of certificates via services such as:

• https://www.ssllabs.com/ssltest/
• https://decoder.link/sslchecker/