Yes, it's in either one of those folders .lock or locks. I think locks. But I checked the other one & there were no files in the other one. So I'm thinking you can delete files that are in both.
I am using 3 machines (Windows, Linux and an Android phone). I logged on Linux, did a sync, then did a sync on the Android app.
During the sync of the Android app, I saw that a .json file disappeared in Dropbox, from
Apps > Joplin > locks.
At the end of said sync, I received the message saying more or less "you cannot synchronize because another device has a lock". But having seen that a lock file was just removed, I did a sync again on android, and it worked.
It looks like there is an issue identifying when a lock is on.
there's a difference here between the sync being stuck and it not synching at all. Mine was syncing. it was just extremely slow