Upon opening an Audacity file recorded on another computer (Windows), I get the pinwheel of death and Audacity won’t respond - needing to Force Quit. This happens on both the local and dropbox directories (where I get the files to).
Try copying or moving the file to a “normal” storage location before attempting to import / open it in Audacity. In the dropbox folder it may still be updating.
It’s invisible to you, but networks have unstable delays, collision detection/resends, and outright missing data errors. To naïve software, that looks like a broken hard drive.