Adobe Acrobat: A running instance of Acrobat has caused an error.

In Acrobat, trying to use a tool or feature results in the following error:

"A running instance of Acrobat has caused an error."

Reason: The error occurs when an already launched instance of Acrobat is busy or in a hang state and you try to perform some operation from outside Acrobat, for example, opening a PDF file by double-clicking.

Please visit Error - A running instance of Acrobat has caused an error (adobe.com) for the solution to resolve. Steps 1-4 is all that is required.