From memory there was a bug in Moneydance 2020 and 2021 up to build 2008. Any chance you are running one of these?
Two solutions upgrade to 2021.2(3095) from https://infinitekind.com/preview you may need to purchase a new licence key at a 50%discount so check Upgrading at https://infinitekind.com/purchase
Or follow this work around.
When using 2020 or 2021 up to build 2008 part of setting up Moneydance Dropbox Folder, if there is not one already, Moneydance should create the folder “.moneydancesync” in Dropbox. It does not, this is the bug. You need to create the folder .moneydancesync manually in the Dropbox Folder. Now try syncing again.
Syncing should not we used as a means for backing up Moneydance’s data set. You should use FILE – PREFERENCES – BACKUPS and set the separate backup folder to use Dropbox.