Have had this issue on several servers both vanilla and modded.
Vanilla Servers: A fresh install, following troubleshooting tips listed above, using A3BRANCH=233780 -beta has eliminated any further Session Lost errors.
Modded Servers: as above plus removing mods that have a shared bikey, if the client or server loads with two mods that share a bikey = Session Lost.
This worked for me and the modsets that our group uses, your results may vary.