Since grav 1.1.13/1.1.14 and admin-plugin 1.2.8/1.2.9 it’s not possible to install plugins via admin anymore.
Uninstall works. Install fails. Though it shows success at first, if you go back to plugins it isn’t listed as installed, on your server there is only an empty folder…
Linked issue: https://github.com/getgrav/grav-plugin-admin/issues/939
I have noticed mentioned problem too. I am using version 1.1.12 of grav.
Fixed now on the server. Should just work… more info: https://github.com/getgrav/grav/issues/1267#issuecomment-274295364