Hi Oliver,
If you are using IW31/IW32 or BAPI_ALM_ORDER_MAINTAIN to create/update work order it will definitely call CO_VB_ORDER_POST module in the transaction update. There could be something else, do you have any upgrades to your system recently or any new Add-on implementations. Sometimes these new Add-ons will cause implicit enhancements to skip unless you generate the Add-on code using SGEN or manually these routines where ever implicit enhancements are implemented. If this is the case, try to activate the module CO_VB_ORDER_POST to make sure function module code is re-generated and will automatically generate the implicit enhancements.
Regards
Syam