This also the case for other content in edit mode.
I checked and that file is actually there and even decided to download a new copy of the theme & overwrite existing files to be sure, but it did not help. I also checked the error log and even for weeks back, no error related to the theme there.
I have no idea when this error could have started, but need to add the site has hardly been modified for a long time.
May be I need to add I use a child theme, but it has not got that file.