This relates to the current stable build that can be downloaded from the App Store. When I grant access to Dropbox, the vast majority of my folders briefly appear and then disappear. I am left with barely any files to choose from. I cannot explain why. Dropbox tells me that Rush has full access. At the moment, Dropbox integration is non-existent for me.
I can send a video of this behaviour if a developer needs to see it but I'd rather not post it publicly here.
Dropbox is getting more and more restrictive in its efforts to coerce people to buy their "pro" version. The free version is now limited to 3 gig of space. It's most likely you're just running out of space and Dropbox isn't letting you save to it.
It's definitely not that. I've had a Pro account for five or six years. This problem has occured very recently. It doesn't relate to saving files. This problem occurs when I am trying to open files to import into Rush.