Does it make a difference if you switch who is hosting? Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin Enhanced Edition\PlayerProfiles\<ProfileName>\Savegames_patch' folder.
Are you using the online lobby to connect or Direct Connection?
Please email supportdos@larian.com, with a description, your latest save and the specs for both systems (or a Dropbox or Google drive link, etc).
For Windows you can use the report.zip file generated by the D:OS EE Analysis Tool. Extract the zip file somewhere and run AnalysisTool.exe. When that loads, browse to the install folder (if it is not automatically detected), optionally run the tests first, and then generate a report (it will be saved in the same location as the Analysis Tool).
The report will contain system and game information, as well as your saved games. To reduce the file size, the host can delete all but one of the saves from the zip (double click the zip file to open it in Windows Explorer, then browse down to the savegames and savegames_patch folders). The client can delete all saves, if they try hosting, or have any from a different playthrough, etc.
For OSX, to get system specs you can use the 'System Report' button from the 'About This Mac' Apple menu item or the 'System Information' app (in the utilities folder). Saves can be zipped by right clicking the folder(s) and selecting Compress, in the folder: ~/Documents/Larian Studios/Divinity Original Sin Enhanced Edition/PlayerProfiles/<ProfileName>/Savegames_patch
Linux is similar, with the saves in the home directory.
