This happens when throwing stuff into speckle that contains arc segments and then trying to read it in revit as floors:
CONVERSION ERRORS
The start and end knots must have multiplicity equal to degree + 1.
Parameter name: knots
changing the degree of the arcs to 2 in rhino fixes it.
Here´s a stream with two examples side by side: Speckle
Hi @emiliano.lupo,
Welcome to the community! Feel free to Introduce yourself 
to the community if you want to! 
This is a limitation on Revit’s side actually and we are working on a workaround atm. Revit can’t create a single closed spline. So if you split it, join it back, create a planarsrf and send it, it should create the floor. 
We are aware this is not the best way for the end users and eventually we want to do this conversion automatically.
Was this arc previously degree 3 with multiple control points before you changed it? If changing it to 2 fixed your problem, this could be an erroneous assumption we are making in Revit in our arc conversions - I can take a look if so 
Hi,
yes, it started as an arc built with the arc command, with degree 3 and multiple control points. Thanks for looking into it
