"Unknown Error" when saving a Premire project directly to a Dropbox folder

I use Dropbox to store most of my files so I can share them easily, so I normaly create the projects directly on a Dropbox sync folder on my PC.

However, since some days ago, I have started getting this error when trying to create or save a .prpoj directly into a Dropbox sync folder:

An unknown error occurred while saving the project.

<Name>.prproj (\\?\D:\Dropbox\<directory>

I believe the error has to do with the "?" in the start of the saving location.

Any ideas on how to fix this issue?

Both Dropbox and Premiere Pro are up to date.