Try having the host load the save in single player and speaking to the Arbiter, then have the others join once combat starts. A couple of people have had an issue with Alexandar's turn not starting properly in multiplay, but it would work in single player.
You can also try using a bedroll and skipping through the dialogue quickly, to see if a change to stats/initiative will make a difference.
If that doesn't help, please email supportdos2@larian.com with your save before the Arbiter (or a Dropbox or Google drive link, etc).
Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin 2\PlayerProfiles\<ProfileName>\Savegames\Story' folder.
In Windows Explorer you can zip file(s) and folder(s) by right clicking on it/them and selecting 'Send To | Compressed (zipped) Folder'.
