"Before deleting anything, download the entire Joplin folder from your dropbox and keep hold of it for potentially aiding to find the root cause. If you right click the root folder in the dropbox web ui, there should be an option to download the whole folder as a zip file."
Regarding this, if you have time either before or after fixing your profile, could you delete the joplin-desktop folder (back it up first if it is a version which is not yet backed up), and open Joplin desktop on a new profile. Then set up file system sync and point it to the unzipped contents of your dropbox joplin folder and post on the forum whether you still get an error or if it syncs successfully (if it still errors, then please post the full log as well)
EDIT: I have now been able to recreate the error using file system sync, but not using a folder. If I create a new text file and save it as hat's.txt to the joplin folder, it breaks the sync with the same error and omits the file extension (if the file was created before the last successful sync, then the error wont occur unless you modify the file first). If you download your joplin folder from dropbox as a zip and then extract the contents to windows, if you then use this search query in windows explorer, do you get any matches?:
filename:*'*
If you do, then either delete or rename the corresponding file on dropbox, depending on whether it is actually a note or not based on the contents, and that might fix the sync