Update: I found an instance of folder/group case mismatch in my project, and fixing it worked (after clearing Derived Data and restarting Xcode). But today, the problem is back :(
UPDATE!! I found an instance of group/folder name case mismatch. After fixing the spelling difference, I deleted Derived Data and restarted Xcode and the problem seems to have gone away!
Update: I found an instance of folder/group case mismatch in my project, and fixing it worked (after clearing Derived Data and restarting Xcode). But today, the problem is back :(
UPDATE!! I found an instance of group/folder name case mismatch. After fixing the spelling difference, I deleted Derived Data and restarted Xcode and the problem seems to have gone away!