YES!!!! We have a breakthrough!!!!!!
Tiger and Cotton spent the night in the pen in their room. They were not soo happy about being confined to the pen, as they knew where they were. We had to weigh down the pen with bricks, to keep them from trying to get out. After moving them into the room, they still behaved as before in the front room, no change.
The night went very well, without any problems. Early this morning, as Cotton was gettting restless in the pen, she started riding him again. Tiger just kept running from her and they were quickly running in circles, but no fighting or aggression. I got up and made them stop as quick as possible.
As soon as it was light outside, it was very hard to keep them in the pen. So we decided to try something risky and put them in their condo. We cleand it with vinegar und put in new bedding. Then we fastened a new board, so they would have more room to lay down in. We made this condo ourselves, while they were still small. That was not that good of an idea, because they got a lot bigger than we expected them to. Especially him. He is the first male we have had and he is huge! He can no longer lay outstretched on the top or middle platform. He can lay down, but not to his full length. Who would have thought, he could get so long. He is about 22 inches (55cm), when he lays all stretched out. Is that normal for a dwarf rabbit?
The condo is 120 x 80 x 40 cm and the two platforms are 40 x 40 cm. On top they have another small extension of 40 cm x 20 cm (with a hole to get up the steps). It's a bit small, but they have time to run around the whole house daily.

My mother had Cotton and I had Tiger when we put them in the condo. We expected a fight or some sort of scuffle, but nothing happened. They started smelling the new platform (board) and then went to their normal snuggle corner. There have been no incidents, no fighting, no pushing, no nothing, except snuggling and grooming.

Their bond seems to be reestablished. We hope that the day goes as it has started. Now are next challenge will be the first run of the house together. We hope this goes okay too but are optimisitic.

The whole bonding process took us about 5 weeks, which seemed a lot longer.
But we finally have two happy bunnies again. Tiger (orange) was always the type to get a little depressed, when he was alone while I am gone for the weekend even though my mother was always there to take care of them.
Thanks for the help and have a happy bunny day!
*happy bunny parents*
PS: I couldn't upload the video.
I have a very strange bonding story, which I would like to share.
We got Tiger and Cotton from a breeder. The woman had two female bunnies (Tiger and Cottons Mommy and one other) in one pen (they were not sisters). Both bunnies had a litter of babies at the same time, in the same nest and would share the nursing of all babies. Tiger was very smart and decided to drink milk from both mommies, so he had always been the biggest of his litter. We thought this was so amazing, that two unaltered females, even with babies, got along so well. At no point did the breeder have to seperate the mothers. This shows us, that in some very rare occasions even females can bind a friendship.