Good question. I suspect that the character limit could be the issue. We are currently using v1.1 of the Twitter API but perhaps upgrading to the new v2 could resolve this... I am opening an issue to check it out.
Unfortunately, I still don't follow. Are you trying to open the assignments from here? If so, I would recommend downloading the xml file from the corresponding assignment directory (this one is an example) and then importing it into NetsBlox using the "Import..." menu item from the file dropdown.
If that doesn't work (and is in fact what you are trying to do), feel free to post screenshots to help show the error you are encountering!