Try updating Internet Explorer to IE11 and see if that helps.

Try reinstalling or updating your NVidia drivers.
Disable NVidia shadowplay and make sure Action! does not run in compatibility mode.

Go to Audio Settings tab and disable "Record system sounds" option and see if issue persists.

In which game does the issue occur?

Probably you are using "selected application" feature and you trying to record application you haven't selected.
Use "Games and Applications" mode to record games and DirectX applications.
Go to Active Screen mode and change the selected application or turn off this feature.

What is your display's resolution and what is the game's resolution?
Open display settings and set DPI to 100%, restart PC, run the game in display's native resolution and see if that helps.

Hello,
try disabling multi-channel audio recording and disable "record microphone to separate audio track" option.

Please report this issue to our technical support via support@mirillis.com

Please report this issue to our technical support via support@mirillis.com

Does Action! crash right away after you start recording or after some time?