You need to be using darcs 2.0 for compatibility with Patch-Tag repositories.
Check your darcs version with $ darcs --version
Update a 1.0 repo to 2.0: $ darcs convert .
You must convert each project in just one place.
If you are the only user of the repo, and are only working on one machine, there should be no complications with converting to darcs 2.
If you are the only user but have multiple machines, consolidate all your repos into one place before converting and pull fresh from the new darcs 2 repo rather than continuing to use the old darcs 1 repo.
If you have a team of users, everybody has to be clued in that the repo is switching to darcs 2 and they need to consolidate all work and start pulling fresh from the new repo... or there could be problems. This is discussed in the manual at http://darcs.net/manual/node7.html.