Site Administration not working

Dear All,
I need help with the following issue.
The installed OJS version is
Both the Administration tab in the Dashboard and in the user drop down menu lead to 404 Not Found and I have no access to the admin options as installing additional language.

The link of the site is, but the administration link is:

I tried to apply the following solution, but it didn’t work:

If I try to manually enter: - I get Access denied

The site administration was functioning normally till recently. I can not connect it to a certain change. The only fix have applied till the moment was: - 4) How can I remove “index.php” from the URLs in OJS?

Could you suggest how do I proceed? We need to install Bulgarian language but at the moment this is impossible.
Thank you for your assistance in advance!