i managed to fix this (for now at least) by resetting all settings in enpass desktop and starting fresh. i hate having to do this because i have to set up all my other settings each time (autolock timeouts, touch ID, etc. etc). i wish those settings could be exported separately from the vault.
meanwhile, i'm not sure how many passwords i added before sync stopped working. that is-- how many passwords have i lost because they weren't synced to the dropbox vault before resetting? i had to reset one password tonight already thanks to this broken dropbox issue: that's at least one lost password.
perhaps i could have recovered this password if i'd been able to merge a backup with the vault on dropbox, but i didn't want to risk losing the rest of my data, or having to spend hours removing duplicates.
the unreliable sync issues (and having to reset all my data to resolve) makes me very concerned about whether i should keep trusting this app to keep my passwords in sync and to not lose them!