[Tom Bunch]

When you configure OmniPlan to use a WebDAV server, we run a few quick tests to see if the server has support for a handful of features that syncing will fail without. It sounds like your server didn't give us an eTag. I'm not familiar with that specific server version bug a quick Google search made me realize that many services ask that ETags be turned off for your particular server. It definitely supports ETags. Can you check your configuration files for lines like:

Header unset ETAG
FileETag None

-Tom