Quit Acrobat.
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.
To resolve the problem, try the following solutions in the order they appear below.
Quit Acrobat.
Do one of the following to open the Task Manager:
Right-click Adobe Acrobat and then click End task. If you see more Acrobat processes in the Background processes list, right-click on each process one by one, and then click End task.
Close Task Manager.
Relaunch Acrobat and see whether the problem is resolved. If not, try the next solution.
Quit Acrobat.
Open the Windows Registry Editor:
Navigate to the following registry key:
HKEY_CLASSES_ROOT\acrobat\shell\open\ddeexec\application
Take a backup: Right-click the Application key and then click Export. In the Export Registry File dialog box, enter a file name and click Save.
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):
Close the Registry Editor by clicking the Close icon (X) at the upper-right corner of the dialog box.
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:
Check whether an Anti-virus program is installed on the machine.
If yes, disable the program. Check whether the problem is resolved.
If the problem is resolved, report the problem to the corresponding vendor.
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