Cannot create streamlines from groundwater heads .vtu-file

When I follow these instructions on either of the two files I get the error message below. It seems to be related to the names of the vector. How do I find out which name would be the correct one for my file?

Traceback (most recent call last):
  File "C:\Program Files\ParaView 5.9.0-RC2-Windows-Python3.8-msvc2017-64bit\bin\Lib\site-packages\paraview\detail\calculator.py", line 203, in execute
    retVal = compute(inputs, expression, ns=variables)
  File "C:\Program Files\ParaView 5.9.0-RC2-Windows-Python3.8-msvc2017-64bit\bin\Lib\site-packages\paraview\detail\calculator.py", line 146, in compute
    retVal = eval(subEx, globals(), mylocals)
  File "<string>", line 1, in <module>
NameError: name 'U' is not defined