Support for multilanguage sites, Internationalization (I18N)

[Connie]

Friends, I have to move a website to another server
before the move everything worked fine.
I use two statements in my template:

this one is in the header and it works like it should:  

<?php get_i18n_navigation(return_page_slug(), 0, 0); ?>

the other statement is for a sidemenu and it is set like this:

<?php get_i18n_navigation(return_page_slug(),1,100); ?>

this one creates permalinks with an additional string "index" which should not be and creates "404":

wrong permalink in sidemenue: https://www.lyriktext.de/index/ingold-uebersetzt/  => 404
correct permalink in sidemenue:  https://www.lyriktext.de/ingold-uebersetzt/  

I found that the default slug of the startpage is added to the sub-pages of the startpage. That's where the "index" in the created slug comes from. But I must get rid of that!!

I have "friendly URLs" activated in the settings and I have cleared the cache and cleared the browser cache
I checked the documentation and can't find the reason

please help me out!

Cheers, Connie

PS: I went through all pages and tried to find a reason for my situation:

- Fancy Urls are active
- custom permalink was entered %parent%/%slug%/

that is why the subpages of the startpage get the "index" in their URL
if I set to "%slug%/" only I had troubles with everything

I set the custom permalink structure empty and now all links are perfect EXCEPT the subpages of the indexpage, because that uses a fixed slug "index" which I can't get rid of