I'm getting this a lot and on 4 different sites. 2 my own personal sites, and the other 2 on client sites who are having major issues.
This is typically happening when we try and upload an image. The image upload progress bar goes all the way to the end and hangs. If i close the lightbox i notice the error. I can wait it out for 5min to get the save button back, which is not ideal.
If i refresh, i notice the image is available to use.
This mostly happened when i upgraded to 3.9. I tried downgrading on one of the sites to 3.8.3 but i'm still getting the issue.
I tried optimizing the database with WP_ALLOW_REPAIR but i'm still getting the same thing.