[TYPO3-german] Typo3 7.6.18 Backend Umlaute teilweise kaputt

[Peter R]

Hallo,
auf dem Live-Server eines Kundens, haben wir das Problem, dass im Backend, der ganz rechte Bereich, immer die Umlaute kaputt sind. (Also der Bereich rechts vom Seitenbaum, wo man die Kontent-Elemente sieht.)
Ich habe herausgefunden, das nur dieser Bereich als ISO (iso-8859-1) ausgeliefert wird.
Ich habe auch schon mal im Typo3-Slack gefragt. 
Hier ein Auszug:

Peter [10:01 AM]
typo3 7.6.18: is there a solution if the frontend shows umlauts correct but backend not? doesn't matter if the text comes from database or language files. e.g. "Neue Ãœbersetzung dieser Seite erstellen". backend works on test system but not live. (de) gibt es ein lösung dafür, wenn im frontend umlaute funktioneren aber im backend nicht? egal ob der text aus der datenbank kommt oder aus sprachdateien. z.b. "Neue Ãœbersetzung dieser Seite erstellen" auf dem testsystem funktioniert alles aber live macht probleme.
i get following error if save umlaut in rte. (de) ich bekomme folgenden fehler, wenn ich umlaute im rte speichere:
These fields of record 974 in table "tt_content" have not been saved correctly: bodytext! The values might have changed due to type casting of the database.

Kevin [10:10 AM]
@peter your database and/or its tables are set to the wrong collation. They are probably set to latin1 but must be set to utf8 (edited)

Peter [10:20 AM]
tables are already set to utf8 (not from me, by default) and umlauts are correct. i guess its not a problem with database. because text from language files is also damaged. maybe some parts from backend are transfered with iso and converted by utf8.

Kevin [10:22 AM]
@peter are there damages in core language labels? Or are there just problems within one specific extension?

Peter [10:29 AM]
translations from core and extensions are damaged.
i could confirm my guess.
live: typo3/index.php?M=help_AboutmodulesAboutmodules
header response: text/html; charset=iso-8859-1
html: text/html; charset=utf-8

test sytem: typo3/index.php?M=help_AboutmodulesAboutmodules
header response: text/html; charset=UTF-8
html: text/html; charset=utf-8

Bernd [10:34 AM]
is the DB-Connection initialized correctly?

Peter [10:42 AM]
in my opinion, yes

Kevin [10:45 AM]
@peter did you alter your webserver configuration to always add the charset=iso-8859-1 header?
because this is not normal for a TYPO3 instance and you mentioned that your test system does not have problems with the charset
so it could be the webserver that forces the wrong charset via htaccess for example

Kevin [10:57 AM]
@peter another idea about the cause of this problem that jumps into my mind right now would be an editor or ftp program, that had set the core PHP files to iso-8859-1 and the webserver is correctly changing the charset of the response to the charset of the PHP files. You could check that very quick by opening a core PHP file in an editor and check the files collation (edited)
I had something similar once with an editor that changed the PHP file collation and it has only been discovered because GIT marked every single file as changed but there where no seeable changes in the diff 

Ende des Auszuges.

Die index.php im typo3-Ordner hab ich geprüft, ob die auch in UTF8 gespeichert ist und das ist sie auch.
Alle anderen aufrufe sind UTF-8 nur die Aufrufe über typo3/index.php?M=* nicht.