Developer having issues in Dev Mode

[schoolsofthough]

We have a user with Developer permissions who cannot do any development in any of our Looker projects.

When he goes into Dev mode, every project is read-only and there's a blue button that says Create Developer Copy that no one has ever seen before. Clicking on that button throws an error: 

"Uh, oh! Something went wrong with developer copy creation. Please try again."

How can we fix this for user? I saw this similar question asked a long time ago, but the update_user API call didn't work for me. And I didn't see any new projects created by user to delete.

[schoolsofthough · Accepted solution]

It was an issue from enabling Labs feature Fast Dev Mode Transition. Disabled it and everything is fine now