Hey Jens, I have the very same issue (assuming it's JWE2 since OP mentions it happening after the CC dlc) and am playing on PC. Every other mode works just fine, only sandbox guests seem to consist of entirely of invisible men and women. Glad the team's aware of it and hope they'll be able to fix this issue soon. It's so weird to have this ghost town of a park even though there are supposedly thousands of people in it.
Sorry to revive this post!
I have found a temporary fix for this. ( PC version, i do not know if it works for PS, try it yourself )
1st make sure the path from the arrival point is well placed / connected.
2nd make sure the path type you are using is the dark grey one, not the pink-ish one ( noticed some bugs with that type of path where a crowd of guests where disappearing off of it).
3rd close the park ( for like 1 min in sped up mode ) then open the park again.
Guests should now show up on the paths.
I do not know if it works for everyone
