Question 

Where I can find FileRemovingDisabled key?

There is a help article related to Creating an ABBYY FlexiCapture back up, in which such key is mentioned. However, it is not clear where it's located.

Answer 

To set up the desired setting:

1. Go to the IIS Manager Console > select the <ServerName>\Sites\Default Web Site\FlexiCapture12\Server node.
2. Double-click Application Settings icon > add the setting key FileRemovingDisabled, where values 1 = true, 0 = false.