Your string is pretty complicated. You have it working in VB, great. If you decide you need to make it work in Python, I suggest dinking around at the Python command prompt. Turns out I did it wrong, see my trial and error here. I fixed the string in my post above - so nice you can edit your posts here!
# what I originally posted, wrong
>>> "\'U:\\rips\\images\\tk50\\tk\'" "\'{}\\co\\*.*\'.format(!BLATT_NR!).lower()"
"'U:\\rips\\images\\tk50\\tk''{}\\co\\*.*'.format(!BLATT_NR!).lower()"
# try again
# Note the python must evaluate to a string expression
# for the CalculateField tool
>>> ("\"U:\\rips\\images\\tk50\\tk"
... "{}\\co\\*.*\".format(!BLATT_NR!).lower()")
'"U:\\rips\\images\\tk50\\tk{}\\co\\*.*".format(!BLATT_NR!).lower()'
