Unknown extension after upgrading CiviCRM in Wordpress
Turn off contributed CiviCRM extensions, (such as PayFlow) before upgrading. Then turn them back on or reinstall them. 

From the guide, "Because of the huge number and ever-changing nature of community contributed Wordpress plugins, CiviCRM cannot guarantee compatibility with contributed plugins. A list of know incompatibilities can be found at WordPress plugins/themes incompatible with CiviCRM." 
Check the compatibility list. 

https://wiki.civicrm.org/confluence/display/CRMDOC/Installing+CiviCRM+for+WordPress