German Umlaute replaced with a questionmark

Hello,

I have the same problem as the previous speakers.
ü ö and ä sometimes with one? shown.
I tried everything that was suggested here

kopano-localize-folder blabla utf8 etc.
Customize / etc / locale
nothing helps …

the only thing that fixes the problem for a short time is a reboot
of the apache webserver.
after the prompt for new application appears, the
Implementation of the umlauts correctly. if the user logs off two to three times
the umlauts are no longer translated correctly.
I have now suspected that my apache copies any files in its cache.
That’s why I’ve tried caching off.
GET /webapp/client/resources/images/loading_m.png HTTP/1.1
Host: 192.168.2.17
User-Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:62.0) Gecko/20100101 Firefox/62.0
Accept: /
Accept-Language: de,en-US;q=0.7,en;q=0.3
Accept-Encoding: gzip, deflate
Referer: http://192.168.2.17/webapp/client/resources/css/external/login.css
Cookie: lang=de_DE.UTF-8; encryption-store-key=a8d7811fa6c6e77b3fb4c11e31b24212; KOPANO_WEBAPP=59mfrdc9d104s080nmp2usjvs4
Connection: keep-alive

HTTP/1.1 200 OK
Date: Mon, 17 Sep 2018 18:32:06 GMT
Server: Apache/2.4.25 (Debian)
Last-Modified: Mon, 13 Aug 2018 14:35:25 GMT
ETag: “c1953-1249-5735201026540”
Accept-Ranges: bytes
Content-Length: 4681
Cache-Control: max-age=0, no-cache, no-store, must-revalidate
Pragma: no-cache
Expires: Wed, 11 Jan 1984 05:00:00 GMT
Keep-Alive: timeout=5, max=100
Connection: Keep-Alive
Content-Type: image/png

but somehow that does not seem to help.
I could reboot the apache every five minutes via a cron job.
This solution is somehow silly.
Sorry if my english is very scary …

Thanks for any help…

I noticed another problem.
If I put a calendar entry with umlauts this becomes
also displayed without umlauts.
A calendar entry consisting only of umlauts, for example üöä
is displayed only briefly and then deleted.
I have now made a trace with the help of Wireshark.
Here you can see that the client sends the word “Gemüse” correctly to the server.
The server but only “Gmse” answer.
I also have screenshots but unfortunately I can not upload them here. #
Here’s a text:
Clientpc to Server:

JavaScript Object Notation: application/json
-Member Key: zarafa
-Member Key: appointmentitemmodule
-Member Key: appointmentitemmodule182
-Member Key: save
-Member Key: props
-Member Key: subject
-String value: Gemüse

Response from server:

JavaScript Object Notation: application/json
-Member Key: zarafa
-Member Key: hierarchynotifier
-Member Key: appointmentlistnotifier
-Member Key: appointmentlistnotifier1
-Member Key: update
-Member Key: item
-Member Key: props
-Member Key: subject
-String value: Gemse

Where can I start looking for the mistake?
Can this be a problem with the database?

0_1537273423594_28265c44-7c40-42ea-8890-4ca1ef922ab9-grafik.png

???

Problem solved :)
An entry in the file / etc / apache2 / envvars helped:
export LANG = “de_DE.UTF-8”