OK, taking up troyb's suggestion, I changed my OFMac preference to a non-secure WebDAV using http:// instead of
https://. The sync from OFMac *appeared* to work, and OFiPhone got further along in the sync than did before. There was no more "untrusted certificate" message when the sync started on the phone, and OFiPhone asked me for my username and password on the server.
Now there's a new error message:
Cannot copy.
Unable to get list of source files from file://localhost/var/mobile/Applications/blahblah/Documents/OmniFocus.ofocus/".
where the blahblah is substituting for a long alphanumeric string.
Looking at the WebDAV server, I see that the sync from OFMac has created a folder called "OmniFocus.ofocus-copy-in-progress-blahblah" which has nothing in it. This is apparently the same problem matt_c has been having (mentioned in another thread today).
In summary, using http instead of https gets around the certificate signing problem, but syncing still doesn't work. Apparently, OFMac is *not* sending the data to the WebDAV server.