This issue is solved. Variables and other settings are loaded in every Qlikview document by a txt-file. A colleague has this txt file customized for his qlikview document without looking to other documents. Thereby certain variables (like sMonthYear) were no longer filled.
Advisable, therefore is: always check the impact of changes on other Qlikview documents before making adjustments in common files (like a txt-file we use)
Note: we use a txt-file so every Qlikview document has the same settings (like colours etc).