Error: Generic Exception: Error: Improperly formatted XML

Hello, I completed the data collection process and uploaded the filled/saved forms onto the server via the android app. However, some finalized forms are not sending. Each time I try, it brings me an error message (Error: Generic Exception: Error: Improperly formatted XML)
How can I rectify this?

Welcome to the community, @Kistalc! Maybe this post discussed previously should help you solve your issue: