There is a bug, that occured when I tried to copy an Object, from which I generated a Symbol into an other document.
I realized, that the object was still marked as symbol after I did the copy. Within the symbols tool window of the new document, the symbol was not listed. When I tried to delete the symbol state, the application crashed immediately.
On restart, Affinity restored the work I did, but took up a lot of cpu power, so my fan started to spin like crazy. Finally, the application crashed again.
On second restart, Affinity designer worked normal.