I think it is because the most up-to-date ChromePlus 1.2.5.0 is based on Chromium development 4.0.206.0.
Following is a note from the Chromium developers about a change to the bookmark sync servers:
http://groups.google.com/group/chromium-dev/browse_thread/thread/5fdf94913d77a064"In recent dev channel updates, we've made some changes to how Chrome syncs
bookmarks with our servers. We're now about to update the servers to match
this new behavior.
If for some reason you are still using a version of Chrome older than 4.0.211.*, [which ChromePlus 1.2.5.0 does]
bookmark sync will stop working and *about:sync* will display OFFLINE_UNUSABLE."