Hello,
My friends and I also experienced this bug and while we wait for a fix have found a workaround - instead of inviting people to the existing save data, start a new game, then load.
1) On the PS4 which has the save data, start an online lobby and invite your friends (or let yourselves be invited). Make sure everyone joins at this point, including split screen players. Then start a new game. Just choose random characters.
2) From the PS4 with the save data, load the save data from the previous co-op save you were playing.
This should hopefully load the old game for all players with no crashes. We tried a few different things and this was the only way to avoid the crash happening when a split screen player joined.
