The document "Main.Storyboard" could not be opened. Unrecognized file content

Just restarting XCode worked for me.

Thanks, XCode.

For me opening the storyboard as "Source code", fixing merge conflicts and then also restarting Xcode did the job.

I just find a solution as below:

Make sure the storyboard extension should start from lower case s. Ex: it should be "Test.storyboard" not "Test.Storyboard"