We're getting this particular error on every page all of a sudden. Not sure when it really started happening.
The exact error is "File temporary://fileA5clzk could not be copied, because the destination directory sites/all/themes/my_theme/assets/css is not configured correctly."
I also see other errors like this:
"The file permissions could not be set on sites/all/themes/my_theme/assets/css." So it seems I need to set the permissions for this CSS folder... except I can't. Are we supposed to be able to modify folder/file permissions on Pantheon? I'm unable to do so.