THere was an error opening this document. THe file cannot be accessed by system.

I am running into this same problem and it just started happening recently.  I have logged out, restarted my computer, and stopped using OneDrive just to see if that would work.

Can someone help me?

Hi @Eric Lennox 

Hope you are doing well

++ Adding to the discussion

Please try the steps below and see if that works for you:

1- For testing, disable Security (Enhanced) under Sandbox Protection. Navigate to:

Edit” > “Preferences“>Select “Security (Enhanced)” on the left pane> Uncheck the “Enable Enhanced Security” and "Protected View" box.

Select “Yes” to the prompt about being sure about making this change.

Click “OK“.
Note: Changing the above security settings could pose security risks. We suggest you to turn it back On once you are done with testing.

2- Delete Dropbox account following the steps Access your Dropbox files in Acrobat/Reader  and re-enable it and check.

~Amal

Are you using the Dropbox feature that only downloads documents when you need them? If so, it is possible that Dropbox is not running on your system, and therefore not handling the on-demand download when Acrobat tries to open such a file. Or, maybe the Dropbox component on your system is stuck. Have you tried to restart your computer?