I just experienced something of the like...
The solution is to (if like us, have it hosted on-prem) to create a binaries folder in die running version of the Html2PdfConverter folder (usually in C:\Program Files\OutSystems\Platform Server\running\HtmlToPdfConverter.<some_identifier>, and grant sufficient access to the OutSystems runtime service user to create and delete files in that folder. I gave my user Modify access, since on the administration page of Html2PdfConverter you can also delete binaries.