So I entereted broken oar grotto (just north of the Solitude on the waterfront) and just as the title says, Serena is unable to leave the cave. I've tried everything but nothing works. I don't know what would have happened if I hadn't already finished the Dawnguard quests, but I reloaded an older save anyways (and lost 1 1/2 worth of time).

This is the second place I've personally encountered this bug, the other being at Labirynthian and Shalidor's Maze (?).

And in case anyone wants to know, I was there retrieving a moon amulet for some Khajit. I had already cleared the place out some time before during another quest and figured "what the hell, get in, get out, done and done". Not so....

How did you mean she was unable to leave the cave? Was she stuck somewhere or just didn't exit when you did? Did you try waiting an hour or so outside to see if that got her out, or did you try fast travelling somewhere to see if that fixed her?

if shes stuck inside, Fus Ro Dah her out the door

Must be a DawnGuard thing as I was actually just in that cave with a follower from the winking Skeever with my new character,
and I had no issues going in or out of course I dont have Dawnguard.

If anything i suggest not fast traveling from inside any caves or areas, only outside in Skyrim, for me at least my game would freeze
if i fast traveled from inside a cave or anywhere else.

Ya I've also been in that cave before with a previous follower (a Housecarl) with no issues.

What I mean is that she would not leave the cave with me. So I tried releasing her, and she dissapears supposedly out the door telling me what she'll meet me wherever, but she never appears at Ft Dawnguard or anywhere else. I let days pass and even did other quests only to return to the grotto to find her still there (now dressed in her 'stock' armor). I've tried getting her out, to no avail. Also, I did not fast travel from anywhere other than outside in Skyrim.

As I said I had the same issue at Labyrinthian where my follower could never leave Shalidor's maze. She's probably still there as I had to take back all my gear from her and return it to my home. This one is a http://elderscrolls.wikia.com/wiki/Shalidorhowever.

And looking at it now, it appears I'm not alone in this http://elderscrolls.wikia.com/wiki/Broken_Oar_Grotto. So beware people.

I go solo

Ya i normally go solo too, but when you use this mod http://skyrim.nexusmods.com/mods/19636
and other mods, you run into enemies that are not leveled with your level, they are like the real world for all you know your fighting a master,
when i started my new character I ran into a few bandits and it must have taken me 10min to kill all of them and considering they way i play for immersion
I needed material to make a sword as I wanted to make everything vs just buy it.

Then at the same time I had no bow, just a pick axe, fights would last like 2-5 min for me, getting a companion really helped cost me 500 gold and I had to save for it,
as I have another mod that reduces the loot too, even they were getting their ass handed to them,
my character is mainly shield and sword so it s a bit tougher, sometimes I use bow depends, Im still using min armor at moment as that is all the material I have found to build what I want.

Couldn't you just fast travel after leaving to make her appear next to you? Even if you're a non fast travel user you could still just fast travel to the Broken Oar entrance.

I also had Serana get stuck in that cave - she wouldn't go near enough to the exit, just stood a way back. I didn't actually notice at first that she wasn't with me and had to go back for her. I managed to gradually push her towards the exit (I hadn't started the MQ so couldn't 'FUS' her, which would have been quicker and easier), when she was right by the exit then she followed me out OK.

I've experienced this with Serana, I eventually got her to leave though. Can't remember how I got her out.

If pc player -> 'moveto player' command in console

This would be the first thing I would try. Leave the cave, stand in front of it and fast travel to the same cave. This is how I get all my followers to "catch up". Sometimes just waiting an hour will work too.

Usually if you leave an area and your follower can't find the way to you he/she will tp to you

I know your pain I lost a pre 1.2 patch master mage thrall because of that horrid place btw pre-patch 1.2 mages were beasts he did more damage with his ice storm spell than my incinerate and much faster too.

I experianced this a long time ago with Lydia at Shalidor's Maze when I was on the Xbox. On the PC some mods can do the same thing to followers usually if there is a problem with the Navmesh.

This recently happened to me with Serana so I parted company with her inside the grotto, she said she would meet me at Darkfall Cave and walked out the cave. I haven't tried to find her as other stuff came up where I didn't want Serana with me but I guess now I will have to try to find where she went to.

Edit: On checking further it seems she stayed in the grotto, if I go back to her, ask her to follow me then tell her to part company she gives a different destination or reply, vanishes but "stays there". Fast travelling doesn't bring her back. The only final thing I have to check is I run a "Followers keep up" Workshop mod. I will disable it and see if it is interfering in any way but I somehow doubt it.

I been there plenty of times with Serena, and hasn't happen to me yet. Although I must say traveling through caves with VL transformation is a pain in the backside to say the least.

I got Serena stuck here too. I guess its only her. I think other followers are OK.

I have my pair of Dead Thralls get stuck in almost every place imaginable, its a headache to keep them from in check honestly. I really need to invest in a pair of heavy duty leashes. Even with two moronic dead thralls I had no troubles in that grotto. Minor issues in Labyrinthia, where they would run off and hide in a corner outside the maze cause they can't navigate the maze. However with Thralls you can just summon familiars so they die, and carry them over tiny pathing glitches.

I did get Serana stuck in a tower in the soul cairn on the stairs, and she absolutly refused to leave the tower. I just shoved her around on the stair case till she was dislodged and eventually she followed me. If that doesn't work, try a sledgehammer.