Crate Hate

We have a 5 month old Mini Dachshund named Oliver. We have had Oliver for about 6 weeks, and he's been doing great on most fronts, including crate training! The typical whining for a few minutes at night time quickly turned into going to sleep. However, recently this past week, he started to regress. There have been a few nights where he gets into this frantic barking and just wont stop. He will go for an hour or more. When I've finally given in and gone down to let him outside, he'll go to the bathroom. After calming down for a few minutes while I hold him, He goes back into his crate and sleeps.

Am i don't anything wrong? How do I help him to self-soothe when he gets into the frantic barking? Should I leave him for longer?

I am starting to work with him more during the day - treats to go in, and come out of crate on command. Leaving him in there for short periods of time while I'm still at home, etc.

Anything else I should consider? Please help!! My wife and I would LOVE a full night's sleep.

Thanks!