Office 365 can't open files in dropbox.

The error indicates Office apps on macOS are having trouble accessing files over the network path (Dropbox), even though the files themselves are fine. The context provides several relevant recovery and workaround patterns for Office on Mac when files won’t open or are reported as inaccessible.

Try these steps in order:

1. Open the files from within Word/Excel instead of Finder
   1. Start Word (or Excel).
   2. Select File > Open.
   3. Use the dialog to browse to the Dropbox folder on the Mac and select the file.
   4. If it still fails, in Word use the Open dropdown and choose Recover Text (for .doc/.docx) and then Open.
   5. If the file opens, immediately use File > Save As and save as a new Word Document (.docx) or Excel Workbook (.xlsx) in Dropbox or on the local disk.
   This follows the documented method for opening blocked or problematic Word files and re‑saving them in current format.
2. Confirm the file type and try another app to rule out format issues
   1. In Finder, right‑click the file.
   2. Select Open With and choose another app (for example, Apple TextEdit for Word files) to verify the file content is accessible.
   3. If it opens in another app, copy the content and paste it into a new Word/Excel document, then save it back to Dropbox.
   This matches the guidance for opening unrecognized formats or when Office cannot open a file that other apps can.
3. Check for Office for Mac updates
   1. Open any Office app (Word, Excel, PowerPoint, or Outlook).
   2. On the Help menu, select Check for Updates.
   3. Turn on Automatically keep Microsoft apps up to date.
   4. Install all available updates.
   5. Restart the Mac and try opening the Dropbox files again from File > Open.
   This addresses known issues after updates where Office apps have trouble opening or saving files.
4. Use OneDrive/online conversion as a workaround for specific files If certain files still won’t open:
   1. In a browser, sign in to OneDrive.
   2. Upload the problematic file from the Dropbox folder.
   3. After upload, select the file so it opens in Word/Excel for the web.
   4. Use File > Save As > Download a copy to save a fresh .docx/.xlsx version back to the Mac (in Dropbox or locally).
   This uses the documented online conversion path to repair or normalize problematic Office files.
5. Verify local disk and temporary file behavior If saving or autosaving to Dropbox triggers errors, test saving to a purely local folder (for example, Desktop). If local saves work but Dropbox ones fail, keep working locally and let Dropbox sync the folder, or move the working copy out of Dropbox until the issue is resolved.

If the problem persists for all Dropbox‑stored Office files even after updates and local tests, collect the exact error text and behavior and contact Microsoft support, as similar Mac Office issues in the context have required deeper troubleshooting of the local Office installation and environment.

References:

• Error: [Filename] uses a file type that is blocked from opening in this version
• You receive an error message when you try to open an Office document
• Open files from the File menu
• There is a serious disk error on when attempting to save, copy and paste or create auto recovery files in Office for Mac