"I receive the error message: error 500 in EOL, what now?

The error message "Error 500: Document Type is not allowed" is given when setting up the connection with Exact using an Exact user that does not have sufficient rights. The connection can only be set up with a so-called "Office user". Presumably, you have used a different type, namely "MyOffice user".

You can ask your accountant for another user, or your accountant can set up the connection themselves using their user via an accountant portal.

You may also consider sending the invoices to Exact via email. To do this, disable the Exact connection and enter the Exact email address for sales invoices in settings > email > accountant's email address."