Last time this addon was updated, it fixed this bug:
"- Fixed a bug that would display a security error if the user returned to the page from paypal"
And after I upgraded, the bug was indeed fixed and all seemed well for a while. However last night a user of my forum reported that they received this bug again, the error was: "Security error occurred. Please press back, refresh the page, and try again" - so it seems users are now seeing this instead of the thank you page.
I'm not sure why the bug would be back, as I haven't changed anything on my forum lately that I can recall. I'm running XF version 1.1.5 still. Checked server error logs but couldn't find anything related unfortunately.