Cannot modify header information – headers already sent during plugin activation

Since the standard redirection works, I figured the conflict must be with TGM plugin activation already hooking to activated_plugin and producing output and thus preventing the redirect…

Therefore the solution was to ensure that the plugin activation function hook was added to an earlier priority than the (silent) default of 10 most probably used by TGM:

add_action( 'activated_plugin', 'activation_redirect', 9 );