slip length (Navier bc) with iterative solver in Stokes equation

Dear Comsol community,

I want to solve the Stokes equation with a Navier bc on one boundary.
(slip velocity = sliplength * du/dn, with n the normal on the boundary
and u the tangential flow velocity)

This works fine with a direct solver, but I did not find any way to get an
iterative solver to converge (2d or 3d).

Neither as viscous slip, nor a "manual" w=wr*sliplength at the boundary works.
It either does not converge or gives errors like "NaN found" or "Inf found".
I tried most iterative solver combinations and studied the Preconditioner Selection Guidelines.

Do you know how to get the Navier bc (special case of Robin bc?) to
work with an iterative solver? Thanks for your help.

EDIT: The default solver for linear elements does work. But since I am interested in forces on boundaries,
I need at least P2+P1 discretization.

Greetings,
Andreas