Hi Bernd,
As you have said,you are facing the problem only in network heuristics but not in location heuristics. This might be because of no LLC's found for that particular data. Check from T-code /SAPAPO/SNPLLC. Click on Display results and find all your product-location combinations.
You find something like this,
Product Location Low-level Code
T-F201 2400 0
T-F201 2500 0
T-F201 2300 1
T-F201 1001 1
'0' being highest priority then '1', '2' goes on.
If you dont find these results, then I would suggest you to run T-code /SAPAPO/SNPLLC, low-level code determination, which is necessary for network heuristics(The low level code determines the order of locations in which the heuristics should run, meaning first heuristics should run for Customer and determines the total demand and passes the same to the connected Depots and then it goes from Depots to Manfacturing Location and subsequently then from Manfacturing Locations to Suppliers).
Post run, you can use either interative planning or T-code /SAPAPO/SNP01 for running network heuristics.
let me know,if you still find issues with network heuristics.
Thanks,
Santosh KB.