Problem with opening horde web mail

Hi to all, i am new on this forum so my question maybe is already asked, have problem with opening HORDE web mail, when i like to open in entering URL [webmail.domain.tld] it opening some file which like to be downloaded

If you ask me for functionality everything is ok , when i connect e-mail on Outlook, Thunderbird e.t.c.

Please advise

here are primary information from server and OS :


Plesk...

Problem with opening horde web mail