I work togheter on an InDesign file with another user. Unfortunately from my side the .idlk (lock)file is not syncing so he can just open it at the same time, change it and we get conflict files. From his side it's working fine (I can see the .idlk file when he has the InDesign document open). I had a chat with technical support but the answer was that we use a different installation of Dropbox (standard vs macOS) somehow and the only way is to use a impractical workaround or reinstall (and resync) the whole Dropbox which is not an option at the moment.
I hope you can fix this or provide another way to collaborate with InDesign files.