Hi again,
here the short version of my problem: I get an error when writing

With ThisWorkbook.Worksheets(sheetname)
If .Cells(delivery_period_row, localloop).Value = deliveryperiod
Then
source_range
= .Range(.Cells(1,localloop), .Cells(max_timeseries_length,
localloop))

I get an error in the last line, stating "object not defined" (or
similar). What do I do wrong?