Hi @Martijn Wingelaar,
thanks for your post. Unfortunately this is a a limitation from Braceworks.
Both the rigs are recognized as 3D structures, and Braceworks requires for 3D structures at least 3 supports that are not in line.
This is the case for the left rig, but not for the right.
The only way to get this system calculating in Vectorworks 2019, is to place a third support on this like you have done for the left rig.
I have filed a Enhancement Request for this in Vectorworks.