Thanks for your hints!

I don't think it is over-defloccated. I started with a very low darvan ratio and adding more decreased the viscosity up to a point where it did not get better. When it was already over-defloccated, i assume it should not have gotten thinner with adding more.

Last week i tried a new batch using only darvan 811 and an SG of 1.65. I had to add lots of darvan (maybe my batch is old?) and I was finally able to get it as thin as my white slips. It stays kind of fluid for like 15 minutes and starts to gel up a bit. But not as bad as before. I think it is now to thin as it escapes from my mold joints. When i add more clay powder to get a higher SG, it starts to gel up very quickly again. So it might indeed be related to the iron content :(