Solidworks 2018 SP4.
I have come across a strange problem with intersection curves in an assembly. Imagine a RIB (boat with tubes around the periphery). I create a plane across the hull and use it as the base plane to insert a new part. In the new part, I create a sketch on that plane, select all the parts involved and then select intersection curve. Works perfectly. I then create the lines, curves etc. and join them to the intersection curves. However when I come to trim the intersection curves, I get a warning box come up saying "the selected intersection curve cannot be used for splitting or trimming". I then have to convert them to construction lines and trace them with a spline. I can then generate the extrude or surface or whatever no problem.
What's strange is that this only occurs on one machine of four, a brand new installation and as far as I am aware, no settings have been changed and I have never seen this behaviour in any other version.
Something must have been changed but I can't see it, any ideas?