I think you shouldn’t uninstall/delete the plugin in WP Admin. And before you change anything make a backup of your site just in case.
I deleted the Polylang folder from plugins via ftp, and then installed 1.7.7.1 through Admin: Plugins->Add Plugins->Upload Plugin, select the zip file.
Upload via ftp didn’t work for me, installing plugin through WP admin did. You can try with that.
Works also. Thanks for quick response!!
