When trying to install/activate plugins on my WordPress site after having to reset them following a host change, I received the following error
Warning: curl_setopt(): supplied argument is not a valid cURL handle resource in ….
This turned out to be an issue with my 1and1 hosting, and how they configure their servers.
To solve the problem I added the following to my .htaccess file:
123 AddType x-mapp-php5 .php AddHandler x-mapp-php5 .php