We had similar trouble with the apple version. tested a few points at the office and everything worked fine. Sent it to the field to collect for a day and it wouldn't sync. We ended up removing the image attachment from every point (save the image to your device first if it isn't already in your gallery). Once the images were removed the offline points synced just fine. We then had to reattach photos after the fact in AGOL.
Not an ideal workaround, but at least it got the field data collected off the device.
Not sure if your error is the same as the one we had with the apple version, but the above might be worth a shot.
I've had similar sync issues with Garmin Monterras, running android, but I was not trying to sync any photos, just two simple tracks and a point. Fairly disconcerting when rolling out a dozen units to collect data in remote areas.
I've had this issue now on several maps, on several platforms, and several devices.