Error message F5A 49 / Item xxx must be corrected in FB60

Hello experts,

our customer uses SAP R/3 4.6C.

In transaction FB60 he enters all necessary information to post a vendor invoice.

When simulating the posting, the system issues an error message in the error log:

"Item 003 must be corrected."

This is the tax item. However, the tax keys and tax accounts are setup correctly and are working in general.

When I go into the line item 003 and press the button "supplement", the document can be posted.

Does anyone have any idea on how to solve this error?

Thanks in advance!

Dirk