Hi Severin,
SAP Support doesnt have any solution for this issue.I would recommend that no changes should be done to AL_BW_REQUEST metadata table.
What I was able to find in our case was that after the upgrade of SAP BW Server from 3.1 to 7.0 in our DEV environment, the target structure was auto generated for e.g a Transactional data structure INDVEJLEVR_PB was changed to INDVEJLEVR_CB. An easiest solution would be to reimport the Master or Transactional target structure and check if there are any changes occurred. We reimported the target and then the job ran without any error.
Hope this helps!!
Regards
Arun Sasi