A running instance of Acrobat has caused an error
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
1) 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:
AcroViewA26 to AcroViewR26
Select OK and close the Registry Editor.
Relaunch Acrobat.