Permissions on library files
Ok, just in case this comes up again, maybe I'll find my own answer :-)

The problem is that modern versions of OS X apply new permissions seemingly at random when problems are encountered. In this case, the OS added a third permission to everything below the home folder, and the permission was "everyone (custom)". Opening it, I found it disallowed writes by me to my own folder.

Suggestions for others; pick a file at random, maybe in your documents, and Get Info. Look at permissions - the first line should be "me", and is likely broken otherwise. Also check the enclosing folder for same.