I think i've found it. After a look into the depbo'ed files i found this:
-the missions you can choose in the "side-mission menu" depends on your rank,
so a different number and names of the side-missions are shown in the menu - maybe my fault was that i started the first side-mission after i liberated all towns and was Colonel yet ?
-in the files, i could only count a total of nine different side-missions, but the "end-trigger" needs >= 10 completed missions ?
Jesus, am i the only one having problems with that ?