This error could be caused by very poor mapping (e.g. 50%) of the structural and fluids interface. To check the mapping, when running the simulation look at the System Coupling output. Scroll down near the top of the file and look for the Solution section, just underneath here you should see the Mapping Summary. Here you will see the percentage of nodes mapped on the FSI interface. If you do see a poor mapping percentage try to change the geometry set-up to allow the geometry of the interface to match exactly/more closely – try to get as close to 100% as possible.