As mentioned in the comments, your question isn't very specific, so I'll try to give you some hints about character encodings, see if you can apply those to your specific case!

Unicode and Encoding

Here's a small primer about encoding. Basically, there are two ways to represent text in Python:

• unicode. You can consider that unicode is the ultimate encoding, you should strive to use it everywhere. In Python 2.x source files, unicode strings look like u'some unicode'.
• str. This is encoded text - to be able to read it, you need to know the encoding (or guess it). In Python 2.x, those strings look like 'some str'.

This changed in Python 3 (unicode is now str and str is now bytes).

How does that play out?

Usually, it's pretty straightforward to ensure that you code uses unicode for its execution, and uses str for I/O:

• Everything you receive is encoded, so you do input_string.decode('encoding') to convert it to unicode.
• Everything you need to output is unicode but needs to be encoded, so you do output_string.encode('encoding').

The most common encodings are cp-1252 on Windows (on US or EU systems), and utf-8 on Linux.

Applying this to your case

I DO have to write äöü in a path, or it will not work

Windows natively uses unicode for file paths and names, so you should actually always use unicode for those.

It DOES have to be an ANSI-"encoded" file, or it will not work

When you write to the file, be sure to always run your output through output.encode('cp1252') (or whatever encoding ANSI would be on your system).

Things like line.write(str.decode('utf-8')) break the funktion of the file

By now you probably realized that:

• If str as indeed an str instance, Python will try to convert it to unicode using the utf-8 encoding, but then try to encode it again (likely in ascii) to write it to the file
• If str is actually an unicode instance, Python will first encode it (likely in ascii, and that will probably crash) to then be able to decode it.

Bottom line is, you need to know if str is unicode, you should encode it. If it's already encoded, don't touch it (or decode it then encode it if the encoding is not the one you want!).

A magical comment at the beginning of the script like # -- coding: iso-8859-1 -- does nothing here (though it is helpful when it comes to the mentioned Metadata and allowed characters in it...)

Not a surprise, this only tells Python what encoding should be used to read your source file so that non-ascii characters are properly recognized.

Oh, and i'm using Python 2.7.3. Third-Party modules dependencies, you know...

Python 3 probably is a big update in terms of unicode and encoding, but that doesn't mean Python 2.x can't make it work!

Will that solve your issue?

You can't be sure, it's possible that the problem lies in the player you're using, not in your code.

Once you output it, you should make sure that your script's output is readable using reference tools (such as Windows Explorer). If it is, but the player still can't open it, you should consider updating to a newer version.

There is no "ANSI"-encoding. "ANSI" means "whatever the default single-byte encoding happens to be on your machine" – the term "ANSI" is inherently ambiguous. This means you must specify an actual encoding when reading the file.

For Windows machines in the Western Europe region, "ANSI" typically refers to Windows-1252. Other regions differ, but also your machine configuration might be different.

Python refers to Windows-1252 as cp1252. If that really is the encoding your file is in depends on the file itself, and can only be found out by looking at it.

Often text editors (not Notepad, real text editors) have an option to interpret a file in various encodings. Pick the one that makes the data look right (pay attention to accented characters) and then find out Python's name for it.

Regarding this definition of ANSI encoding:

ANSI encoding is a slightly generic term used to refer to the standard code page on a system, usually Windows. It is more properly referred to as Windows-1252 on Western/U.S. systems.

You can try:

"\x98".encode("cp1252")

But, Python doesn’t allow it so, you can use ISO Latin1 instead:

"\x98".encode("iso_8859_1")
# b'\x98'

There's no ansi encoding in Python Standard Encodings.

Choose appropriate encodings from following link: Standard Encodings

Try this

#read input file
with codecs.open('USERS.CSV', 'r', encoding = 'latin-1') as file:
lines = file.read()  

#write output file
with codecs.open('1_UserPython.CSV', 'w', encoding = 'utf_8_sig') as file:
file.write(lines)

Why don't you read the file and write it as UTF-8? You can do that in Python.

#to support encodings
import codecs

#read input file
with codecs.open(path, 'r', encoding = 'utf8') as file:
  lines = file.read()

#write output file
with codecs.open(path, 'w', encoding = 'utf8') as file:
  file.write(lines)

ANSI is not a character encoding (in common parlance it refers to certain escape sequences, though it's of course the acronym for the American National Standard Institute). You can set the encoding in Notepad++ (and check what encoding you're using) -- hopefully utf-8, because that's a universal encoding (lets you represent any Unicode point). You build unicode from your utf-8 encoded text with an explicit decode method call, or you read the file as unicode with a codecs.open (both require you to specify your encoding name -- again, hopefully 'utf8').