Hi!
I have battled the 'Not well formatted JSON Stream' error lately and I cant seem to find a solution.
I have heard that maybe it is an issue with SharePoint but this never used to happen for the list I have been bringing in for years and whose data has not been edited for some time.
I also came across the attached messaging and now I am wondering if that is the issue?
SharePoint List Input Tool (alteryx.com)
I'm afraid I never used the previous tool but, just on the off-chance, have you tried using the new SharePoint input tools?
These work pretty well and should be fairly simple to set up (though not sure if this will be similar to previous version of tool or not).