I'm going to rewrite my bug so it fits on the list:
Type of Bug: General
Bug: I can't return to the Soul Cairn through the portal. The only thing that happens is that the portal drains my health like the first time you try to enter.
Additional notes: I completed the Dawnguard questline on the Dawnguard side, I never tried to return there while I was doing the Dawnguard quests (I first tried to return there after I had completed the last quest), Serana was following me when I tried to enter again, I tried as both human and vampire (nothing worked), I'm in level 50 and play on expert.
Just to clarify: when it came time to enter the Soul Cairn, did you have Serana turn you into a VL or did you soul trap yourself? I have a feeling this bug only presents itself if you decided to become a VL and then cured yourself. I ran into this same problem in my playthrough. I played on the Dawnguard and had Serana turn me into a VL to get through the Soul Cairn, and then I got cured so I could complete the questline (Isran won't talk to you as a vampire lord which is an expected drawback I suppose but a righteous pain nonetheless). I reclaimed my lycanthropy and kept playing. Later, I thought about going back to see what I had missed and I ran into the same problem you did, couldn't re-enter. However, when I had Serana bite me and turn me into a Vampire Lord, I WAS able to go back inside. I'm curious to see if anyone else was able to.
On a side note, I think it is incredibly irritating that one of the main draws of the DLC would have such an obvious problem with it. If you are going to give people the option to be a vampire lord even after siding with the Dawnguard, it would be nice for the functionality to not be crippling to your progress through the quest as well as having numerous bugs associated with it. I mean my game has a glitch where I am now forcibly turned into a vampire randomly, and it seems to be connected to my switching back and forth. I've had to cure my vampirism like 7 times. Not only that, the game still thinks I'm a werewolf, as my "days as a werewolf" statistic just passed the 100 mark and continues to climb.
I'm grateful to not have any game breaking glitches for certain, but all this little crap can snowball over time, and it is seriously starting to become annoying. I trust in Todd and all the talented people at Bethesda though. Skyrim is an awesome game and I don't doubt they will fix this stuff, but I hope it is soon. Having played through Morrowind and Oblivion, I remember a lot of glitches (heck you could kill quest related folks in Morrowind and I remember the old Clavicus Vile glitch from Oblivion, which I didn't have an internet connection to patch) but I don't remember there being a vast swarm of them like there seems to be in Skyrim.