A phyton script that completely removes a special block from an XML file. It removes the block if a tag in that block is empty. In this example, it looks for ContextNumber and checks if it is empty.

removeTransactionFromXML.py

import xml.etree.ElementTree as ET

zahl = 0

# XML load
tree = ET.parse('input.xml')
root = tree.getroot()

# Removing the part
for task in root.findall('./Task'):
    for transaction in task.findall('./Transaction'):
        for issue in transaction.findall('./Issue'):
            contextDatas = issue.find('./ContextDatas')
            if contextDatas is not None and contextDatas.find('./ContextNumber').text == None:
                task.remove(transaction)
                zahl += 1

print(str(zahl) + " Transaction Blocks were removed.")

# XML save
tree.write('output.xml', encoding='utf-8', xml_declaration=True)