In the current build (Release 8.1 build 20051214_2200) there appears to be a compatibility issue between the two types of structural beam families. There are two templates to make them as shown in the image below.
When you have both types of families loaded, place a Complex and Trusses.rfa. Then sketch a Beams and Braces.rfa family. Then try to place a new truss instance. You should get this error message. The only workaround I have settled on so far is to use a Generic Model category instead which is unfortunate. I've submitted this to support and am waiting to see if they can reproduce the error.
Hope this saves someone a little troubleshooting time.