Invalid certificate on client-application

I set up a Cloudflare tunnel to connect a client-side application with a server-side application.
The server-side application consists of a server and webapplication-manager.

I can access the web application through https and http. https://manager.domain.com
however, the communication between the server-application and the client-side application can only be established via HTTP http://server.domain.com
The client shows an error message when trying to connect via https stating that the server was not connectable because it was using an invalid certificate, and the identity could not be verified. X_509_V_OK : 0.

Is there some setting Iā€™m missing?

I already installed the Cloudflare-root certificate on both machines.

the tunnel is set up as follows:

manager.domain.com ā†’ http://localhost:22000
server.domain.com ā†’ http://localhost:26000