[R-SIG-Mac] Please help! Package Installer doesn't work.

[Luisa Bahnemann]

Hello ,
I’ve only installed R (R 3.4.0 GUI 1.70 El Capitan build (7338))
a few days ago . Now every time I try to install a package I can’t even see the list of packages and R completely freezes.
Then it shows this :

Warnung: unable to access index for repository https://mirrors.cicku.me/CRAN/bin/macosx/el-capitan/contrib/3.4:
  kann URL 'https://mirrors.cicku.me/CRAN/bin/macosx/el-capitan/contrib/3.4/PACKAGES' nicht öffnen

Could you please help me?
I already reinstalled but it didn’t help.

Thankyou !
Luisa Bahnemann