Error Messages

Suddenly getting these error messages. Nothing has been changed on the website for some hours but just visiting it now and these errors are displaying at the top of the screen:

Warning: session_start(): Unable to clear session lock record in /var/sites/t/[thewebsite]/public_html/wp-content/plugins/wc-multivendor-membership/core/class-wcfmvm.php on line 58

Warning: session_start(): Failed to read session data: memcached (path: xx.xxx.xx.xx) in /var/sites/t/[thewebsite]/public_html/wp-content/plugins/wc-multivendor-membership/core/class-wcfmvm.php on line 58

Fatal error: Uncaught Exception: Hybridauth requires the use of ‘session_start()’ at the start of your script, which appears to be disabled. in /var/sites/t/[thewebsite]/public_html/wp-content/plugins/wc-frontend-manager-ultimate/includes/libs/Hybrid/Storage.php:22 Stack trace: #0 /var/sites/t/[thewebsite]/public_html/wp-content/plugins/wc-frontend-manager-ultimate/includes/libs/Hybrid/Auth.php(100): Hybrid_Storage->__construct() #1 /var/sites/t/[thewebsite]/public_html/wp-content/plugins/wc-frontend-manager-ultimate/includes/libs/Hybrid/Auth.php(39): Hybrid_Auth::initialize(Array) #2 /var/sites/t/[thewebsite]/public_html/wp-content/plugins/wc-frontend-manager-ultimate/core/class-wcfmu-vendor-verification.php(118): Hybrid_Auth->__construct(Array) #3 /var/sites/t/[thewebsite]/public_html/wp-content/plugins/wc-frontend-manager-ultimate/core/class-wcfmu-vendor-verification.php(24): WCFMu_Vendor_Verification->wcfm_vendor_auth_requests() #4 /var/sites/t/[thewebsite]/public_html/wp-content/plugins/wc-fro in /var/sites/t/[thewebsite]/public_html/wp-content/plugins/wc-frontend-manager-ultimate/includes/libs/Hybrid/Storage.php on line 22

Do you know what could be causing this?