This error started with iOS 26.5. Here are the fixes:
1. Use Share > Save Video instead of the three-dot menu: In Dropbox, open the video, tap the Share icon, then choose Save Video. This uses a different API path that bypasses the iOS 26.5 validation bug.
2. Export to Files app first: In Dropbox, tap the three dots on the video and choose Export. Save it to the Files app (On My iPad). Then open Files, long-press the video, and choose Save to Photos.
3. Update Dropbox: Open the App Store and update Dropbox. Recent versions include a patch for iOS 26.5 compatibility.
4. Use VLC or Documents by Readdle: Import the video into one of these apps from Dropbox, then use it to save to Photos.
5. Update iOS: Go to Settings > General > Software Update. Apple may have patched this in iOS 26.5.1 or later.
The Export to Files approach (step 2) is the most reliable immediate workaround for your editing workflow.