Your "Sessions" files might be corrupted. Try this:

1. Organize your tabs across your Workspaces.

2. In each Workspace, right-click the Opera window and select "Save all tabs as Speed Dial folder". Rename the folders to match the Workspaces for easy reference.

3. Go to opera://about, copy the Profile path, navigate to it and open the "Sessions" folder.

4. Close Opera completely. Then, back up and delete all files inside the "Sessions" folder.
This will cause your Workspaces to reset.

5. Open Opera, go through your Workspaces, open a Speed Dial tab, right-click the folder matching the Workspace, and select "Open all in new tabs". Repeat for each Workspace.
(Keep the Speed Dial folders saved as a backup, just in case.)