Invoicing – AffiliateWP Integration causes fatal WordPress error

Hi there,

some days ago I have installed the AffliateWP plugin as well as the Invoicing – AffiliateWP Integration. At first sight things went fine. Before yesterday the Integration plugin was deactivated and I needed to re-insert the license key. The moment I activated the key my site went blank white with the message that there is a fatal wordpress error. The error is gone as soon as I deactivate the Invoicing – AffiliateWP Integration plugin.

Today the same, as soon as the integration plugin is activated there is a fatal error.

What must I do to get the integration plugin going?

Many thanks in advance for your support
Birgit