Sextante decodes a vector layer as point, linestring or polygon depending to the first checked geometry of the layer. This has an ambiguous effect on mixed geometries layers (point+linestring for instance) that OJ can handle.
The problem is easly solveble on class es.unex.sextante.openjump.core.OpenJUMPVectorLayer , part of OJ binding (since I am porting new Sextante libraries from GvSIG CE to OpenJUMP, I opened this bug ticket only to keep trace of solvable/solved bug)
Solved on Sextante OJ Binding nov 2016b for next OJ real.