Hi @michal.sowa,
You only have five segments less than 0.8mm, so easily cleaned up manually.
To find the short lengths, copy the polylines to a new layer with a contrasting colour and explode it. Hide your original layer. Use @Helvetosaur’s SelCrvsByLength script from WISH: Select By Length to identify the short curves (invert the resulting selection and delete so you can see the wood for the trees).
Jeremy
p.s. You have a polyline that crosses itself near the top of your object.