Quick question for you guys, what version of macOS are you running? I realized this has started happening after I updated to 11.2.3. Might be a coincidence though. I just tested mine and it's working. I'm on macOS 11.2.2. I'm running Xcode 12.3 with Rosetta and 12.4 without Rosetta.

You mean with Rosetta you can edit .xib files? I haven't tried with .xib files, but I can edit a .storyboard file with Rosetta which does not open without Rosetta (shows "Internal Error"). EDIT: Sorry I had this the other way around. The .storyboard edits without Rosetta, and does not open with Rosetta. It will only compile with Rosetta.