Before everyone jumps in, no this isn't the bug a lot of people have had, where you can't complete the Thieves Guild Loud and Clear quest right at the end.
The one I'm encountering is very odd, and I've done heaps of searching, but I seem to be the only one!
When I get to Goldenglow and meet/pickpocket/kill Aringoth, the safe key isn't in his inventory, but there is a cellar key. Strange, but not a big deal as I can pick the lock to the safe downstairs. But when I do that, there's only gold in there, not the other bit that is needed for the quest.
The weird thing is that the quest arrows are pointing to Aringoth and the safe, but the items are not there, thus I can't complete the quest.
I've tried earlier saves, bringing/not bringing a companion, going/not going through the optional sewer, setting/not setting the beehives on fire first.
I have done Mjoll the Lioness' quest to return her sword. As she doesn't like the Thieves Guild, does that mean I can't do their quests?
Any help would be greatly appreciated, other than restart from the very beginning!

Thanks.