Close the error message and follow these steps:

1. Go to: C:\Users\%USERNAME%\AppData\Roaming\SDL\SDL Trados Studio\14 for Studio 2017
            or C:\Users\%USERNAME%\AppData\Roaming\SDL\SDL Trados Studio\15 for Studio 2019
            or  C:\Users\%USERNAME%\AppData\Roaming\SDL\SDL Trados Studio \Studio16 for Studio 2021

2. Rename the Plugins folder
3. Run an SDL Trados Studio repair (Repairing your installation of SDL Trados Studio) from "Control Panel\Programs\Programs and Features" (optional)
4. Start SDL Trados Studio  again

If the issue persists, please also  go to:
                 C:\Documents and Settings\%USERNAME%\Local Settings\Application Data\SDL\SDL Trados Studio\14 for Studio 2017
            or C:\Documents and Settings\%USERNAME%\Local Settings\Application Data\SDL\SDL Trados Studio\15 for Studio 2019
            or C:\Documents and Settings\%USERNAME%\Local Settings\Application Data\SDL\SDL Trados Studio\16 for Studio 2021
and rename the Plugins folder.