[plugin: members] installed, but not displayed plugin list

Hi,

I tried to install the ‘members’ plugin. The installation went fine and I clicked on ‘activate’. After this the plugin was not visible in the plugin list (/wp-admin/plugins.php). I tried to install an other plugin, but the same problem arose.

When trying to reinstall the plugin, I got the notification that the folder named /wp-admin/plugins/members.php does already exist. When visiting this URL I get an error that the URL can not be found on the server.

I am working on a single site account(saw that other similar problems occured with multi-site accounts).

Does anyone know how to fix this? Or where else to find the installed plugin?
Thanks!