btw, I am using dovecot v. 2.2.27-3

<0x0x561e04d12c30[SOGoSieveManager]> WARNING: Could not delete Sieve script - continuing...: {RawResponse = "{ok = 0; }"; result = 0; }

That is only a warning and is harmless.
You always get it when sieve is used without a prior sieve script.
It has nothing to do with your problem.

Please be sure you have the correct SOPE packages installed from inverse repo.

SOPE 4.9 is installed from the repository

Please install apt-show-versions and check with:
apt-show-versions | egrep 'sogo|sope|gnustep'

Done. See the output below:

[root@mail ~]# apt-show-versions | egrep 'sogo|sope|gnustep'
gnustep-base-common:all/stretch 1.24.9-3.1 uptodate
gnustep-base-runtime:amd64/stretch 1.24.9-3.1 uptodate
gnustep-common:amd64/stretch 2.6.8-2.1 uptodate
gnustep-make:all/stretch 2.6.8-2.1 uptodate
libgnustep-base1.24:amd64/stretch 1.24.9-3.1 uptodate
libsope-appserver4.9:amd64/stretch 4.9.r1664.20170721 uptodate
libsope-core4.9:amd64/stretch 4.9.r1664.20170721 uptodate
libsope-gdl1-4.9:amd64/stretch 4.9.r1664.20170721 uptodate
libsope-ldap4.9:amd64/stretch 4.9.r1664.20170721 uptodate
libsope-mime4.9:amd64/stretch 4.9.r1664.20170721 uptodate
libsope-xml4.9:amd64/stretch 4.9.r1664.20170721 uptodate
sogo:amd64/stretch 3.2.10.20170822-1 uptodate
sogo-activesync:amd64/stretch 3.2.10.20170822-1 uptodate
sogo-common:all/stretch 3.2.6-2 uptodate
sope4.9-gdl1-mysql:amd64/stretch 4.9.r1664.20170721 uptodate
sope4.9-libxmlsaxdriver:amd64/stretch 4.9.r1664.20170721 uptodate

sogo-common:all/stretch 3.2.6-2 uptodate

That one is from debian repo.
You should remove that.

Everything else seems OK.

Do you get errors in your mysql log?

done.
No error at all.

With "no errors at all", I ment - that there is no mysql error or something else, but preferences are still not saving...

Have you restarted SOGo after removing that package?

Have you cleared entirely your browser's cache?

Now, it's a little bit different.
Preferences are saved now, except the language.

If you choose the language "English" at the login page, everything will get loaded in english. You can then set the language to German (for example) in the preferences and the page will reload to german language. After logout and re-login, it will be english again, if english is choosen at the login page (even german should have been saved before?)

If you change the language to german before the login, everything will be german.

In my opinion, it should be possible, to save a desired language, independently from the language choosen at the login-page?

Please open a separate issue for this.