Hello,
I got the 1.4.10 version of VMinvoice and tried to automatically send invoices.
All my VM emails to show onwner and buyes are send, but the automatic sending from VMInvoice does not work.

1.
I tried to setup a con job as explained in the manual.
Nothing happend.

2.
The moment I set the plg_vminvoice, active to test out if that would be the solution, I got the error written above, the moment the iDeal (=bank) payment is handled correctly and that php returns to my url.
In this case the joomla emailing was set to PHP mailer.
Did not try this with joomla in SENDMAIL.

Please Jan, advise what to do.

Where from does this error arise from? Could you please post it in full.

The reason is most likely that there are 2 or more plugins instanciating (or defining) the PHPMailer class, which causes this error. I believe it should not be caused by VM Invoice, as it does not instantiate this method directly but it fully relies on Joomla's API.