Can you run
SYNCEVOLUTION_DEBUG=1 syncevolution --daemon=no loglevel=4 <your operation>
and attach the output?

I think I know what is going on:
- SyncEvolution proactively sends Basic authentication,
  to avoid extra round-trips if the server accepts that.
- The server sends a permanent error, instead of
  asking for some other way of authentication.
- SyncEvolution (or rather, libneon) gives up.

What kind of CalDAV server do you run? And how did you change the configuration
to enforce Digest Auth.