Well I used 24bit TGA for the diffuse map and 32bit TGA for the normal map.
=> no stencil shadows
=> normal map with decent specularity

If I remove the normal map or reduce it to 24 bit
=> stencil shadows
=> no normal map or plastic look

Engine version is last public beta (6.506) and my testobjects are just simple boxes (closed), placed on terrain and blocks.

Well, doing some research it turned out, I´m not the first one who has a problem with the automatic transparency thing in A6: link

I guess it has something to do with this, because the manual recommends, that for stencil shadows the entities must not be transparent.
Maybe these lines of code could solve this?

ZWriteEnable=True;
AlphaBlendEnable=False;
AlphaTestEnable=True;

These, of course, are just some ideas of a shader noob ...