That error can have several causes, so I'd start by checking the linked table itself. Open Linked Table Manager, select that table, and try refreshing its link. Also verify that the back-end file opens normally by itself and that you can open the table directly from the front end.

If the link refresh fails, remove that one linked table and link it again. If it still happens, the problem may be with the back-end file, its location, or permissions rather than the form, report, or query.

Run through the troubleshooting checklist too: 599cd.com/Trouble

Let us know where the back end is stored, whether other users can open that table, and exactly what happens when you try to refresh or recreate the link.

The original Back end is on our company server. Its data comes from an ODBC to our MRP software. It is read only. We Access to see and analyze data in more user-friendly format. There are a few tables that are read-write. There are multiple users but only one person has write permission. I use task scheduler to daily make a copy of the back end and put it in my drop box. I use this view data and work on making test forms and reports without messing up anybody's front end. All this to say, I manually copied the backend and put it in my drop box and voila all is good. I've been using this technic for years with no issue. Everything is easy once you know the answer.

Running a Microsoft Access backend directly from a folder managed by a cloud synchronization service such as Dropbox or OneDrive is a serious reliability risk. The Jet/ACE database engine relies on file-system locking and coordinated read/write operations. The .ldb / .laccdb files support Access's locking mechanisms, but cloud synchronization tools are not a substitute for a proper shared network file system. Their synchronization processes can introduce conflicts, delays, or competing copies of the database, potentially resulting in data loss or corruption.

Richard has also covered this subject in his videos and issued various warnings about it. The fact that a setup has worked for years without an apparent problem does not make it safe. Read-only usage may reduce some write-related risks, but it does not establish that the configuration is reliable.
Check these out
Four Different Ways to Share Your Microsoft Access Database Online. Use Access Remotely Anywhere.
Using Microsoft Access with File Sharing Services: Dropbox, OneDrive, Google Drive, etc.

Thank you, Alkan, for your valuable input. The working database is on the server. I am working with a copy backend, basically a "scratchpad" to test forms, reports and queries.

The concern here isn't whether your test copy works, the core issue is the underlying assumption that a Microsoft Access database can be safely managed inside a cloud synchronization folder like Dropbox. Access relies heavily on file system level locking and coordinated file I/O operations. Dropbox synchronization is not a substitute for a proper shared network file system, nor does it provide the necessary guarantees for concurrent database access. Depending on how the file is accessed and synced, conflicts, inconsistent copies, or outright database corruption can occur. Using this approach for years without an apparent issue does not eliminate the risk, it simply means the specific failure conditions have not occurred yet.
The key distinction is cloud storage is useful for backing up or distributing closed database files, but actively working with an Access database inside a synchronized folder even just for testing forms and reports is a completely different story.
You need an isolated development copy on a local drive in a native Windows environment to run your tests, otherwise, the reliability of your test results themselves becomes questionable. When even a minor synchronization hiccup throws errors like Unrecognized Database Format in this thread of yours it is worth reconsidering keeping test backends in cloud sync services.
As a MVP like Richard, along with many veteran developers in this forum and personally as a developer who has been working with Access since version 1.0, we strongly advise against this approach. Ultimately, the choice is yours.

Alkan's right. Dropbox is fine for storing a closed backup copy, but I wouldn't open and actively work with an Access back end while it lives in a synced Dropbox folder.

For your scratchpad setup, copy the back end from Dropbox to a normal local folder first, such as C:\Access\TestData\, and link your test front end to that local copy. When you're done, close Access and then copy the file back to Dropbox if you want it archived or available elsewhere.

That may also explain why manually making a fresh copy cleared the error. Glad you got it sorted out.

For the record, I've told this story in a couple of my videos. I used to have an office that I'd drive to every day, and my database was a single ACCDB file. I'd copy it to my Google Drive. By the time I finished my 20-30-minute drive to the office, the file would be synced. I could then copy it from Google Drive down to my local C drive, and then do the same thing backwards when I went home.

I would never actually run the database from inside Google Drive, because you're just begging for corruption. I found that out the hard way a couple of times. You can use it as a staging location to copy it back and forth, or for backups. Zip it first if anything, and then put it there, but don't run it from Google Drive.