Same as JamesRoss -- downgraded to 1.6.6 and now no error.
However, I did run into something... maybe just my relative new-ness to CodePlex, but I can't believe it takes this long for a branch/tag to show up.
I tried (twice) to create a branch/tag -- once from the HEAD, once from a specific version -- both times it appeared to create ok (no errors & normal message about WC not connected to the branch), BUT I see no changeset in CodePlex.
However, when I tried to create a 3rd tag set from the working copy, I get an error saying I have to update the WC -- and when I do it's 'updated' to a version that's non-visible in CodePlex and which doesn't show in the SVN repo browser either.
Nothing has shown up for about 3 hours now. Any ideas?