one side says up to date, other side says syncing

I didn’t find any other exact answer.

So, I have two machines syncing, let’s call them M and R

From rachine R, syncthing says about machine M: up to date.

On machine M, syncthing says about machine R: Syncyng, with 39,384 items out of sync. This never evolves.

How can that be?

Difficult to say without more specific information.

First of all, if possible, please post screenshots showing the Web GUI on both sides. Secondly, make sure that you’re running the newest or at least recent versions of Syncthing on all devices. Thirdly, make sure there are no permission errors that are preventing the files from syncing.

If 2 parties are syncing to each other, every party is finished as soon as it has received and processed the changes from the other side.

Your example: R says it is up to date with M indicates it has received all updates from M. While M is stil 39384 items out of sync. This is possible because M needs to receive a lot of data from R.

It is possible that the sync is configured one way. For example the photo sync from android is configured that way by default. It only sends updates, it never receives updates.