I found a very strange bug with synchronizing attachments. It only happens with certain specific attachments, and only on Android phones, with feature services that have their credentials stored in ArcGIS Online. From around 60, or more, JPG images I tested, I found two that cause the error. The images are the same size as some other images I tested and that synchronized without problems, around 2 or 3 Mb.
I submitted the bug to our local ESRI support, and they put it through to Redlands, so I am hoping it will be solved. In the meantime I thought I might post it here, since it is a potentially very annoying bug that can be very hard to find.
The error says: An error occurred while synchronizing edits
Service = https://utiliy.arcgis.com/usrsvcs/services/....<rest service details>../FeatureServer: Server Error