Hi @Vedran_Hasanagic, welcome.
This is the error in the log:
Query error: Column 'item_tax_rate_id' cannot be null -
Invalid query:
INSERT INTO `ip_invoice_items` (
`invoice_id`, `item_product_id`, `item_name`, `item_quantity`,
`item_price`, `item_discount_amount`, `item_tax_rate_id`,
`item_description`, `item_product_unit_id`, `item_order`,
`item_product_unit`, `item_date_added`
) VALUES (
'redacted', 'redacted', 'redacted',
'redacted', 'redacted', NULL,
**NULL**, 'redacted', NULL,
redacted, NULL, 'redacted'
)
Can you see if you accidentally have forgotten to fill the tax rate for your invoice?