Dear Lars,
This is the first time that you post in this forum, but before posting things, first search the forum before asking help.
Your issue has already been solved before January 1,
There is a new 9% VAT code implemented and the “concept btw aangife” already works with this new percentage since Manager Version 18.12.21.
If your not on this version or newer, download the latest version
Read this thread on the forum and you find al necessary details.