\f
is interpreted as the page brake and is replaced with hex code 0xC
. You should remove leading backslash from path.
tree = ET.parse("folder" + ID +'.xml')
And if you use backslash inside strings it can be escaped like this \\
EDIT
When you work with paths it is better to use os.path module:
import os
...
tree = ET.parse(os.path.join('folder', ID + '.xml'))