No second author shown

Dear Riza,
The problem is that no second author can be added to an article. After saving, the second disappears and only the first remains. This way articles are not listed at the second authors’ pages (not equivalent to your plugin’s author pages!) For a while I also had the problem described in these two tickets:

https://ww.wp.xz.cn/support/topic/polylang-guest-authors-automatically-created-for-each-language/

https://ww.wp.xz.cn/support/topic/plugin-constantly-creates-guest-authors/

I have disabled guest authors and now this seems to be solved – but no second author can be added.

This whole problem persists since the url format was changed from something.hu/hungarianpage or /englishpage to something.hu/hu/hungarianpage and something.hu/en/englishpage. This change was necessary because of other issues, but ever since adding a second autor is not possible (only through guest authors – but they are also not listed on the authors’ pages). So the problem might be around the url of the authors but I can’t figure out, how. I hope you can help.
Thank you for your help in advance!

The page I need help with: [log in to see the link]