Dear Blackstridaaustria,
thanks for your comments.
Of course a completely loose bolt is not useful at all. But we are just talking about one third of a full turn since the tube would not move more than that from unfolding to folding position. My fear is more that a repeated unloosing and tightening of the bolt will limit its lifetime due to a wear-out of the thread.
But anyway I now added a thin stainless steel washer to armor the tube and protect it from further damage and will order some Loctite to get the bolt fixed again. That is then close to your solution.
Furthermore I mounted a strong magnet at the upper frame protector to fix the lower tube with it in the folded position to limit the force on the rear hinge just with this plastic clip that Strida offers for the Evo. This works quite well.