Hi Kamal,
hi Mukthar,
thank you for your response.
I was able to identify the cause of the problem. In the transaction OKG3 there is a setting "redistribution" in the block "Cancell. / Meth. of Appt". We have an "A" as a selection and this selection ensures that the results analysis values are apportioned to the individual line ids via the method of apportionment (and not to only 3 line ids as I assumed before).
And due to the fact, that we have had 3 line ids which didn't have an appropriated assignments, the Res. for Unrealized Costs haven't been posted correctly to FI.
I have maintained the missing assignment and now the Reserves for Unrealized Costs can be posted correctly.
Thank you again for your hint. Have a nice day.