This is generally caused by an issue with the application (Word or Excel) not responding to communication from the browser so there may be an issue with Microsoft Office.
If you are seeing a "484" error when opening Excel documents, Excel may be set to "Ignore other applications that use Dynamic Data Exchange (DDE)." You will want to make sure that this option is NOT enabled or you will not be able to open Excel files (.xls, .xlsx) from outside of Excel.
Here is what the error message would look like:
And here is the setting in Excel:
We've also seen cases where this setting is unchecked (as it should be) yet users still see a 484 error when opening documents from NetDocuments. If this is the case you may need to set https://vault.netvoyage.com as a Trusted Site in IE: