I found the issue, in the Adobe docs this code snippet is wrong. The quotes need to be updated :-)
Test an email image Adbox
<img src=“https://{clientcode}.tt.omtrdc.net/m2/{clientcode}/ubox/image?
mbox={email_header}&
mboxDefault={http%3A%2F%2Fwww.domain.com%2Fheader.jpg}&
mboxXDomain=disabled&
mboxSession={123456}&
mboxPC={123456}” border=“0"/>
it should actually be:
<img src="https://{clientcode}.tt.omtrdc.net/m2/{clientcode}/ubox/image?
mbox={email_header}&
mboxDefault={http%3A%2F%2Fwww.domain.com%2Fheader.jpg}&
mboxXDomain=disabled&
mboxSession={123456}&
mboxPC={123456}" border="0"/>