Issue with REGION command - region_issue.dwg (0/1)

Hi there,
I hope anybody here could help me with the problem I'm having concerning the REGION command. I have a closed contour comprised of
Lines, Arcs, Polylines and a 2D Polyline (see the attached dwg). Running the REGION command on the contour results in the error:
"Open Objects must be coplanar."
Which suggests not all objects are in the same plane.
Analyzing the entities however, shows there are no Z-values at all and the BOUNDARY command works just fine.
Googling and looking on ADN for this issue yielded no helpful results...
What's really strange is that converting all Lines to Polylines solves the issue. (Unfortunately, this is not a solution for me as I am developing an automated process which cannot go around converting other peoples entities.)
I have further isolated the problem to the lines composing the 'indentation' of the contour. I can however not find anything special about those Lines...
Has anybody had similar experiences with REGION? Can anybody reproduce the problem with the attached drawing? Is there anything 'wrong' with the entities composing the contour?
In other words.. HELP! :)
TIA, Wouter
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