Check the currency definition of the corresponding journal itself or the currency definition of the contra-account defined in the journal.

An accounting entry (posting) assigns an amount to an account and simultaneously to a contra-account (offset account / "Gegenkonto" in german) .

Do not explicitely define currencies to accounts and journals, as long as you do not exactly know what you are doing and why it is needed (it is only needed in rare cases).

Without knowing what a "contra-account" is, we set all Accounts in the Journal to EUR, to no avail :(

One of the strange things is that Accounting > Settings don't save, instead throw following error.

File "/home/v9devblaze/odoo/openerp/addons/account/models/res_config.py", line 169, in onchange_company_id

self.default_sale_tax_id = isinstance(taxes_id, list) and taxes_id[0] or taxes_id

IndexError: list index out of range

Consequently the Accounting > Settings > Default Company Currency loads empty.

Update 1:

We managed to resolve the empty Default company currency issue, which seems to be unrelated to the invoice-validation problem, by removing the following user-default of taxes_id (from Settings > Technical > User-defined Default, in Dev-Mode):

Separately..

Going by this video from an older OpenERP version https://youtu.be/1EZ2HwJ7j80, there used to be such a field called Secondary Currency. Does the concept of Secondary Currency for Journal Accounts still exist in odoo9, especially for single-currency setups?

If this is the "Contra-Account" as Erwin Trevisan suggested, where can Contra-Accounts be manipulated?

Thank you in advance..

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Update 2

Following Erwin's advise, we removed the mention of EUR currency on the related Accounts (specifically 3: 101200 Accounts Receivable, 111200 Current Liabilities, 200000 Product Sales). That got rid of the original error about "Secondary currency..".