I dont know if I just didnt do It right, but once I close a project, I cant change it anymore. I can open It but the brushes just dont do anything, no matte what I try. Is there somebody who ran into a similar problem, who jan tell me how to change this?
(Id suggest making a savetycopy of all your nomad folders with files and custom stuff first) then what can you do when you open the file? Can you rotate it? Select tools? Maybe your cameraview is selected in the objectlist (if you made one) sometimes when that happens you have to manually click on an object in the list or doubleclicking works i think, maybe you have lock selection enabled? but its kinda hard to tell from guessing 
Ps: might be better to ask in bugreport
’ I can select tools, rotate it, but when I try to add clay for example, nothing happens….
Its better if you ask in the „bugs“ forum section and a screenshot/video of the workspace with stats, if its not a tool or filesetting blocking it (mask enabled, 0 opacity, brushdrag instead of stroke etc) for you it might be a new bug or something not so common. Do you have enough memory? By default you should be able to edit the model right away.