I have been able to edit files that have been previously edited in Lightroom, and this includes some DNGs. The problem has come with exporting to Photoshop for further editing and printing. What frequently happens is that PS opens the files in ACR, which is useless at that point. Evidently, LR's xmp files create a problem when they are exported from C1 to PS.
The problem that you describe is different, but it may relate to the same basic issue. Phase One is well aware of the xmp glitch and have promised a fix with the next update, which should be out soon. In the meantime, I would suggest that you submit your problem officially to Phase One Support on their website. They may already have a simple fix available for you.