Yeah, I also thought about the clutch piston sealing!

Thinking about all this 24h a day I just remember again:

The day I got that trans (that was 50miles ago) I exchanged the oil pan with my B&M oil pan with my temp sensor. When removing the pan I saw EXACTLY this black little pieces on the bottom of the pan like on the picture of the filter! I didn't feel them, so I don't know if they where soft or hard, but the size was the same, it wasn't just a varnish/paste like it's everywhere in the oil now. It where 3-5 little black pieces! That trans was dyno tested before I got it!
I asked the builder because I was worried when I saw that, but I was told this is normal when using Kevlar bands. Now that I saw that the bands are orange I know that this pieces didn't come off the bands! Maybe from the clutches, but the clutches I saw till now are also more orange/red than black!
Could that have been the begining of the end???

I deinstalled both cooler lines and flushed them well with carb cleaner, that stuff cleans ALL, it kills your hands too!
I also flushed the original 26" radiator from right to left till just clean carb cleaner came out and then did the same thing from left to right! I almost choked from that stuff!
I didn't and still didn't use any extra oil cooler, the weather is very cool (60-68F) and I wanted to install the cooler this week. So that can't be the problem.
I drove the car 2 weeks ago the first time with the new trans around 10miles, it hardly got warm! Then 2 days later I had this first time the reverse problem that didn't work. (the car was just running 5min and it was still pretty cool)
And few days later I drove the other 40miles without a problem. And yesterday I drove 3 miles and then reverse didn't work and at the end no forward gear worked...

Tomorrow I will try to get that transmission out and hope nobody comes up with some money in their hands wanting to buy that car! I am so d"*%"*% frustrated that I might sell all this crap!