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.
Change the value of the key from AcroViewA20 to AcroViewR20 (where the last two digits or numbers depend on the Acrobat version installed on your computer; For example, for Acrobat 2020, it will be 20):
- Double-click the Default under Name. The Edit String dialog box is displayed.
- In the Value data field, change the value to AcroViewR20 and then click OK.
- To automatically update from Acrobat, choose Help > Check for updates. Follow the steps in the Updater window to download and install the latest updates.
Relaunch Acrobat and see whether the problem is resolved. If not, try the next solution.
Acrobat or the resources it depends upon could be damaged. Try repairing Acrobat as follows:
- Run repair from within Acrobat: Choose Help > Repair Installation.