Resolve 15 "davinci resolve failed to save project"

I was working on a project for a day and then got the message, 'failed to save project'.

Research on line suggested that I may have run out of disk space, but I have a hundred gigs left. Another suggested that I open a new database, but I still get the same issue. Also tried 'save as', same problem. Am not sure what to do anymore. There is no prompt for me to upgrade my database. I am stuck. I can no longer save my projects. I tried to downgrade to Resolve 14, but all the databases are no longer accessible. Even back ups. 15 seems to have done something to them. This is disconcerting.

I have the same issue. On Davinci Resolve Studio 15 public beta. Windows 10. "Save as" works but "Save" project prompts error.

@MichaelWang I guess that is why they call it a "public beta", it is not fit for projects because it is obviously unstable. Best to downgrade to the more stable version until they get Resolve 15 (non-beta).