p.s.: I solved the currency error (it was due to an invoice preference using the € symbol instead of EUR.
Opening the invoice list is indeed much faster than before: it now takes 8 seconds.
Creating a new invoice still takes nearly 40 seconds.
But even with the latest version of master_2019.2 in a clean install, the comma used as a thousands separator in the amount still gets passed on to the database insert query and causes the same error.