Remote Desktop Manager shows an "Invalid certificate" error message when attempting to update from a version earlier than 2023.2.34.
As a security precaution, Remote Desktop Manager includes a list of accepted certificates to ensure the updates are distributed by Devolutions (a method named certificate pinning).

To solve this issue, manually download ans install the update via your browser.
- In the Remote Desktop Manager ribbon, go to Help – Check for updates.  
- In the Update available tab that opens, click Download, then Download update using your default browser. You browser opens and the download should start.  
- When the download is complete, click on the downloaded file and follow the installation process.