No, I think the issue is that something goes wrong between the browser and web player app when the latter tries to pass the finish URL to the former, i.e. the problem you're running into likely is the same as before (when trying to pass the subject ID to Qualtrics). As before, one way to avoid that issue should be to set the finish URL directly in the script. For example, you run a file called explicit.iqx at the end of each of your two <batch>es:
/ subjects = (1 of 2)
/ file = "AMP_PosNeg.iqx"
/ file = "SMP_RichPoor.iqx"/ file = "Explicit.iqx"
/ subjects = (2 of 2)
/ file = "SMP_RichPoor.iqx"
/ file = "AMP_PosNeg.iqx"/ file = "Explicit.iqx"
If you change explicit.iqx to read
<survey Explicit>/ onblockbegin = [
defaults.finishpage = text.finishurl.item.1;
/ pages = [1 = Mediator; 2=WA1; 3=WA2; 4=AR1; 5=AR2; 6=AR3; 7=SRpage1; 8=SRpage2; 9=SRpage3; 10=Dem1; 11=Dem2; 12=Dem3; 13=Dem4; 14=Dem5]
/ screencapture = false
/ showbackbutton = false
/ showpagenumbers = false
/ showquestionnumbers = false
/ nextbuttonposition = (50,90)
/ finishlabel = "Finish"
/ items = ("https://www.millisecond.com/web/finish5.aspx?IQConfirmationCode=<%script.subjectid%>")
that should sidestep any potential issue. I'm attaching the modified explicit.iqx file below. Can you give it a try with this one? Thanks!