Thanks! I tried to find *.tmp files within the mods but they seem to be clean.
I found a possible solution from another post to start game -> load current game -> reload that game again. Supposedly this for some reason makes it possible to save. Seems to work, but with Skyrim you never know :)
My Save Games are already at 23Mb, that seems large too. But I doubt my added mods are responsible for that.