content for money – 404 error after paying

Hi, Content for Money worked perfectly until yesterday but now, due maybe to a new installed block download plugin, after the person has paid through paypal, the page of my site to wich he comes back does not show the hidden content but instead a 404 page not found error.
I uninstalled and deleted the block download plugin, then unisntalled and installed again content or money and so on with all the plugins, emptied caches…but nothing goes back as it was!
The 404 error advise is shown in the wordpress page, not a blanck page.
Thanks for helping!

https://wordpress.org/plugins/content-for-money/