I am also having trouble with this and getting the same message. I am also including the long version of the error.
No valid Facebook session for this app detected, private events will NOT be synced.
Fatal error: Uncaught exception ‘Facebook\FacebookAuthorizationException’ with message ‘Invalid OAuth access token signature.’ in /home/content/a2pnexwpnas04_data02/90/3301190/html/wp-content/plugins/eme-sync-facebook-events/Facebook/facebook-php-sdk-v4-4.0.23/src/Facebook/FacebookRequestException.php:104 Stack trace: #0 /home/content/a2pnexwpnas04_data02/90/3301190/html/wp-content/plugins/eme-sync-facebook-events/Facebook/facebook-php-sdk-v4-4.0.23/src/Facebook/FacebookRequest.php(280): Facebook\FacebookRequestException::create(‘{“error”:{“mess…’, Object(stdClass), 400) #1 /home/content/a2pnexwpnas04_data02/90/3301190/html/wp-content/plugins/eme-sync-facebook-events/eme-sync-facebook-events.php(185): Facebook\FacebookRequest->execute() #2 /home/content/a2pnexwpnas04_data02/90/3301190/html/wp-content/plugins/eme-sync-facebook-events/eme-sync-facebook-events.php(504): eme_sfe_sync_events(‘username edited’, ‘password edited’, Array) #3 [internal function]: eme_sfe_options_page(”) #4 /home/content/a2pnexwpnas04_data02/9 in /home/content/a2pnexwpnas04_data02/90/3301190/html/wp-content/plugins/eme-sync-facebook-events/Facebook/facebook-php-sdk-v4-4.0.23/src/Facebook/FacebookRequestException.php on line 104