Running instance of Acrobat caused an error

Last updated on Aug 16, 2025

Learn how to troubleshoot a running instance error in Adobe Acrobat on Windows.

You may encounter the following error when you try to open a PDF or use a tool while another instance of Acrobat on Windows is already running:

A running instance of Acrobat has caused an error

Unresponsive background processes

Quit Acrobat.

Press Ctrl + Shift + Esc to open Task Manager

In the Processes tab, locate Adobe Acrobat under Apps and Background Processes.

Right-click each Adobe Acrobat process and select End task.

Close Task Manager and relaunch Acrobat.

Incorrect registry entry

Quit Acrobat.

Press Windows + R to open Run

Type regedit and press Enter.

Navigate to HKEY_CLASSES_ROOT\acrobat\shell\open\ddeexec\application

Right-click the application folder and select Export. Save the backup file.

Double-click Default in the right pane. In the Value data field, change the value from:

AcroViewA20 to AcroViewR20

Select OK and close the Registry Editor.

Relaunch Acrobat.

Outdated Acrobat version

Open Acrobat.

Select Menu > Help > Check for updates.

Install available updates and restart Acrobat.

Corrupt installation file

Open Acrobat

Select Menu > Help > Repair installation

Follow the on-screen instructions to repair the files.   

Restart your system after the repair is complete.