Thank you. This should be fixed in the next update. Unfortunately, major updates during Early Access do not maintain saved game compatibility (the release will also not maintain save compatibility with EA saves) and there is no way to fix this problem from the save itself.
The problem is caused in co-op games by an issue keeping track of player characters during character creation, if one or more players selects a character and starts going through the customization, then changes their mind and selects a different character.
This bug does block the end section, but in Early Access you can not get past the first act in any case.
