This issue was caused by the upgrade of the plugin W3 Total Cache.
I upgraded it to latest 0.9.5 just now and found my website did not work anymore, and from the error log I got the same error as yours.
Below is a temporary workaround, hope it is helpful to you.
1. login the server directly and go to the WordPress Plugin folder(wp-content/plugins), then to wptouch/core folder
2. make a backup of file class-cache-smash.php first
3. edit the file class-cache-smash.php
line 96: $w3_config = new W3_Config( true );
to: $w3_config = w3_instance(‘W3_Config’);
line 99: if ( $w3_config->get_cache_option( ‘pgcache.enabled’ ) ) {
to: if ( $w3_config->get_boolean( ‘pgcache.enabled’ ) ) {
line 101: $rejected_user_agents = $w3_config->get_cache_option( ‘pgcache.reject.ua’ );
to $rejected_user_agents = $w3_config->get_array( ‘pgcache.reject.ua’ );
then try to activate the ‘WPtouch Mobile Plugin’ again.
Hope the next update will fix this issue.