I too am experiencing this issue. The database (and all associated data) has been upgraded to newer versions of Invoice Plane multiple times, however the last upgrade seems to have broken things.
I too do not use the tax system. All tax values are set to None.
The errors I am getting are:
Severity: Notice --> Undefined property: CI::$mdl_invoice_tax_rates ~/public_html/vendor/codeigniter/framework/system/core/Model.php 77
Severity: Error --> Call to a member function where() on null ~/public_html/application/modules/invoices/models/Mdl_invoices.php 252
InvoicePlane 1.5.4, Debian Linux, Apache2, PHP 5.6.30, MySQL
One particular major problem is that every CRON run causes Invoice Plane to create one new invoice with no items and a value of $0 every time it runs. If multiple recurring invoices are pending, it only does the first recurring invoice it encounters and then stops (erroring out and not going to the next recurring invoice). Before this problem, my last valid invoice ID was 966, the invoice IDs have since incremented to 2272, all of which are invalid invoices.