FWIW, perhaps try my one step Shortcut.
https://www.icloud.com/shortcuts/2bd1a1455ace43e1a8f49bcbe8cf1864
If it works, perhaps try rebuilding yours based on this one.
If it doesn’t work maybe try signing out of Dropbox on Shortcuts. From the Save file action with a Dropbox selected, tap on the greyed out icons at the top of the Dropbox logo (top right). Select sign out in the dialog box that appears. Then sign back in and re-try.
There was a similar’ish bug for the share sheet a while back to do with images; so i guess I should also say, please ensure that your version of Shortcuts is up to date.
Similarly, because this is an extension thing, I think there could be some memory limitations that could come into effect. I’m assuming that you are using a relatively modern iOS device (i.e. plenty of available memory), but please let us know if that is not the case,
Beyond that I can only think of app reinstalls as an option; but that would seem an extreme step and I would hope we can figure something else out before trying that.