1. I'm using the default coupon integration.
2. updating the database in the update/migration section didn't work
3. the problem seems to occur in the german version of virtuemart only. they forgot to rewrite the coupon code to be set to db entry c014h_virtuemart_products_de_de instead of c014h_virtuemart_products_en_gb
4. i copied c014h_virtuemart_products_de_de table to a new table c014h_virtuemart_products_en_gb. even if this works, it could break somewhere in the VM installation.
5. next error is 1146 Table 'AxelSchmallenbach_website_shop.#__virtuemart_categories_en_gb' doesn't exist
6. it's better to re-write the code in VM from the VM developer team than this problem. Even if I manually copy all german tables to english tables, will it work?