Is the check valve spring loaded? If so, what is it's cracking pressure (CP)? If CP > 100 mbar, then flow is zero. Is the check valve screwed, flanged or welded connections?
I can help get you part of the way to the answer with what we know already:
42 m3/h = 700 LPM.
In DN50 (2" Sch. 40 = 2.067" ID), v = 17.7 ft/sec (5.4 m/sec)
For a pressure drop of 1 bar (14.5 psi), K = 6.9 (dimensionless). See attached .pdf.
If the check valve is not spring loaded, it has a relatively large resistance compared to 2" swing check. What type check valve is it? A lift check?
You see, we are guessing, because you have failed to give complete, detailed information.