EDIT: oops, that wasn't it, also. My State Cache simply didn't enable the correct texturing unit on bind, when that texture already was bound (this fix also fixed different other problems).
I just found it - just too simple: obviously (I'm on NVidia) the texturing unit our RTT texture is bound to needs to be reinitialized after resize (NOT ON INITIAL SIZING). Unbinding the texture and rebinding it when needed again did the job.
P.S.: I'm working on a State-Cache that uses all available texturing units - that's why this popped up: the texture was never unbound as my examples use much less textures than there are units... (so no texture gets unbound unless deleted).