“There was a problem saving the project. Not all recent edits could be saved. The project will now close.”
It showed me this and when I re-opened the project, I’d lost days of revisions. Why didn’t it warn me it was no longer auto-saving earlier? I am devastated.
(Scrivener version 3.5)
Scrivener would have shown the message as soon as the issue occurred. If you’re missing more material than that, the most likely reason is that you inadvertently opened an older version of the project.
One common cause of the error would be if the project were moved while it was open. For instance, if you used the Save As command to save to an external drive, then removed the external drive. That would also explain the “missing” material, since the most current version of the project would be the one on the now-detached external drive.
So my suggestion would be to look around for other copies of the project, both on your main drive and elsewhere. You can locate all projects on your computer by searching for .scriv files in Finder.
You can also locate Scrivener’s automatic backups by going to Scrivener → Preferences → Backups and opening the backup folder in Finder.
I keep my project opened at all times, so not sure how that could’ve happened.
if the project were moved while it was open
I didn’t touch the files or any other programs on my computer though :\
This is a long shot, but this happened while I had my VPN connected. I wonder if it had a weird interaction with the Dropbox folder, which is where my project is stored. (Although I would’ve thought that would give me more issues with syncing rather than the project itself saving)
And unfortunately, since I only generate a backup when I close the project, and I rarely ever close the project, I don’t have any backups of this work. Lesson learned in keeping Scrivener opened between editing sessions 
Oof. Yeah. That’s a problem.
First, if there isn’t enough local disk space, that would (obviously) keep Scrivener from saving. It might also cause Dropbox to “helpfully” upload files to their own server. Which, if Scrivener was open at the time (which it apparently was), would be, again, an example of moving the project while it was open.
If the issue is that your Dropbox account is out of space, that shouldn’t affect Scrivener’s ability to save locally. However, I don’t know how Dropbox handles an “account full” condition, and I wouldn’t want to use critical data to find out.
Do you have a Time Machine or other automatic external backup?
So, it looks like it was my local storage (being out of space) causing Dropbox to disable syncing (makes sense). I made some space and that began working again.
And I can assume the lack of local storage was why the save issues popped up. I don’t know if Scrivener could detect why it can’t save and show a message with that information (“Couldn’t save because project was moved” vs “Couldn’t save because of lack of storage space”). If it’s possible, it would make finding the underlying issue easier.
I also don’t know why I experienced what felt like a long delay between making edits and receiving a “can’t save” warning, I don’t know how frequently the program auto-saves (or retries?). Just an observation for your devs.
As I said, Scrivener triggered the message as soon as it encountered the issue.
The auto-save interval is defined in the Scrivener → Settings → General → Saving tab. Certainly Scrivener would have saved – successfully – at some point while you were away from your computer. Based on your description of the problem, my hypothesis is that you were victimized by some evil combination of Dropbox – possibly including inability to access the Dropbox server due to your VPN – and the lack of space on the local drive. Scrivener reported the error it saw as soon as it occurred, but the damage to your data may have already been done.
As for a more useful error message, unfortunately Scrivener simply sends a “save this file” command to the operating system, which reports back either “success” or “failure.” Scrivener can’t tell you why the failure occurred because it doesn’t know.
