Synchronization issue, stuck loading at 99% when friends join my server

[Feetus]

When i start a new save game everyone can join just fine, but after playing for a while once i restart the server everyone gets stuck loading at 99% when joining. I noticed a lot of people having this issue but haven't found a solution so far. Port 10823 is forwarded properly in my router, there's an exception for the game in my windows firewall for both public and private networks. All playing on PC btw.