Hi Matthias,
If you read my document about Static report - you will see that everything is very simple 
1. Your column axis is static - meaning - static number of columns!
2. You have to create members with member recognition.
3. You have to use correct Excel formulas to generate member id's that are referenced in EPMOlapMemberO functions.
4. You have to upgrade your EPM!!!!!!!!!!!!!!!!!!!!!!!! SP15 is very old and may have expansion bug.
Vadim