Open an issue on KOReader's GitHub with more details (and a crash log) about the dropbox crash.
Because there's approximately a 0% chance of being able to piggy-back on Nickel's own syncing. Because it's no longer running

.
(Running a random script that may happen to do a sync (i.e., rclone), yes, that's been sitting on my TODO list for a while now, but that's it).