Hello,
I have found and solved the issue.
I went into the Facebook catalog and filtered products with price 0.00. There i did find 36 products marked as with a problem. When i studied these products i figured out that all from day one was registered in my shop as simple products, then over the years they have been changed to variable products and new variants added.
Also some products was wrong because of SKU that has been changed and this change has not been added to facebook catalog.
I was able to delete all faulty products in catalog, but some that had wrong id because of SKU had been changed i was not able to delete so i had to change SKU in woocommerce and then i was able to delete product from catalog via woocommerce.
I have re-synced all products again and problem seems to be solved. Wierd is that i have not seen these products in ads with price 0.00 before, just now for the last 2-3 weeks.
Anyway, problem is solved. And i now understand that i need to delete products from catalog before i do changes as mentioned above.
Brgds
Rune