The tables in my invoice template are having borders around them.
When I edit the tempate using the GUI the changes are not saved. This is also the case when I change the HTML tag of the table from border=1 to border=0.

Where is this config saved (in a file or in the database) so I can edit the template in another way and is this a know bug?

Hello,

We have not this problem with Joomla Content Editor (JCE).

If you want to edit template via database, you can find config in #__vminvoice_config. There are columns: template_body, template_footer, template_header, template_items which should be used for invoice template.