Hello, we can’t use odoo11 on mobile phones (both android/apple).

Error (translated): "The certificate of your server is not trusted. Connection not possible."

Pecularity: We unrolled new server certificates last summer. After, on all newly integrated mobile phones in our network, the odoo app has not been usable. Only on mobile phones that we have had in usage already, it is possible to use the app.

To solve the problem, we updated our nginx-configuration and again reinstalled the wildcard certificates a few weeks ago. The problem still remains.
As our database is hosted on premise (and we have version 11), we do not get official support anymore. We excluded the error source VPN. As the access via mobile phone browser is possible, we think that the app itself is the problem.

An assumption: Could there be a second certificate storage (we have not found) exclusively for the app? This could explain that our new, but unlike certificates do not match the requested one.
Help will be very appreciated!