'Copy Document' function not working with prepaid invoice

lospet

Hello forum,

I am using NAV 4.0 SP2

The following process steps are subject to my issue:
1. Create Invoice e.g No. 01
2. From invoice 01 create a prepaid invoice PI01 ('Function' ->'Create Prepaid Invoice')
3. Copy invoice 01 Header and Sales Lines to the prepaid invoice PI01 ('Function' ->'Copy document)
4. Post prepaid invoice PI01
5. Meanwhile customer pays according to amount of the prepaid invoice PI01
6. Post Invoice PI01
7. When posting PI01 NAV automatically reverses the prepaid invoice PI01 entries (neg. sign) and posts the invoice entries to the defined accounts

And this is my problem:
When using the 'copy document' function NAV does not copy the sales line. I receive a warning that if I continue, the existing saleslines will be deleted or overwtitten. I press OK and nothing happens except the copy of the header.
Further on it is very strange that when moving to the Prepaid specification invoice I can see the sales line that I was trying to copy.

So my question is why does the 'copy document' function not work when copying documents into prepaid invoices? Copying into all other types of documents works fine.

Hope somebody can give me some useful hint or advice.

Thank you in advance

Best regards,
lospet