Portable: Lost remote control (mouse/KB) when working in Session

This issue occures between 2 windows computers running Teamviewer portable Version 15.1 (Dec 2019) and both PC's running on admin level.

During a session everything is working as expected, but when trying to start a program, the local and remote mouse and keyboard are "disconnected" and only the host can continue. (the computer that is requesting the help). After completing some steps, the remote mouse/keyboard are back in business.

I provided both sides with the default full access control in the settings. Restarted teamviewer several times, but still no progress. Any ideas I can try ?