This particular problem happens when there is slow internet connection or network congestion, as the XML is file is not read completely and henceforth > it throws exception stating to upload proper XML. Trying after sometime may resolve this problem. Kindly follow the following steps :

Set the Internet Explorer settings to Low. Go to Control Panel — Java — General — Settings Uncheck the “Keep Temporary Files on my computer” and click the  “Delete Files” button and press OK.

Close your existing browser and open a new browser and upload your xml file again. Moreover, use only Internet Explorer or Google Chrome as your browser. If the problem persist, kindly contact the ASK call center at ask@incometaxindia.gov.in.