I can reproduce the error in the latest version. I am not sure that it "means" anything other than there is a bug.
As a workaround, use one of the other methods for loading lobs (any of the other choices in that dropdown).
You can check all rows in the grid by doing a CTRL+A to select all. Then select the ones that you don't want to rebuild and hit the delete key to remove them from the grid. Then click the green triangle to rebuild.
Thank you for reporting the bug. I will fix this for version 15.1.
-John