After updating to PRTG 25.1.104, the PRTG core server does not start anymore, and the log contains the following message:

Signature of \Program Files(x86)\PRTG Network Monitor\32 bit\PRTG Server.exe is not valid

What can I do to resolve the issue?

This article applies as of PRTG 25.1.104

Trusted root certificate after updating to PRTG 25.1.104

As of PRTG 25.1.104, a new trusted certificate authority signs the binary of PRTG. As a result, the code signing signature verifications fails on systems on which the following trust chain in the trusted root certificates of the Windows certificate store is missing:

This causes the PRTG core server to not start in some cases. You can resolve this issue if you add the trusted (root) certificates. You can either use the Microsoft Root Certificate Program or download the certificate from Entrust. 

Microsoft Root Certificate Program

To use the Microsoft Root Certificate Program to add the certificates, follow the instructions here: Updating List of Trusted Root Certificates in Windows.

Download from Entrust

You can also download the following certificates and import the missing ones to the certificate store on the PRTG core server system and probe system: