@bsdwerbeagentur I just tested the convert options with 2.8 alpha 2 and it seems to work for me. I successfully converted a Curve > Circle to a mesh. I also successfully converted a mesh object with modifiers down to a modifier-less result – Essentially collapsing and applying object modifiers which is a common [hack] use for the conversion options.
But when I tried converting the default cube to a curve I got nothing. I think because that’s not a valid conversion? I’m wondering if you’re experiencing something similar.