HTML_QuickForm::validate() Error

Question

I'm using CiviCRM 5.4.1 and Wordpress 4.9.9.

When registering for an event, the user hits the Continue button and gets this:

In the civi log I get this:

Feb 15 15:13:53  [info] $Fatal Error Details = Array
(
    [callback] => Array
    (
        [0] => CRM_Core_Error
        [1] => handle
    )

    [code] => -1
    [message] => QuickForm Error: unknown error
    [mode] => 16
    [debug_info] => Form rule callback returned invalid value in 
       HTML_QuickForm::validate()
    [type] => HTML_QuickForm_Error
    [user_info] => Form rule callback returned invalid value in 
       HTML_QuickForm::validate()
    [to_string] => [html_quickform_error: message="unknown error" code=-1 
    mode=callback callback=CRM_Core_Error::handle prefix="QuickForm Error: " 
    info="Form rule callback returned invalid value in 
    HTML_QuickForm::validate()"]
)

Feb 15 15:13:53  [info] $backTrace = #0 /var/www/vhosts/<site>/httpdocs/wp- 
content/plugins/civicrm/civicrm/CRM/Core/Error.php(232): 
CRM_Core_Error::backtrace("backTrace", TRUE)
#1 /var/www/vhosts/<site>/httpdocs/wp- 
    content/plugins/civicrm/civicrm/packages/PEAR.php(921): 
    CRM_Core_Error::handle(Object(HTML_QuickForm_Error))
#2 /var/www/vhosts/<site>/httpdocs/wp- 
    content/plugins/civicrm/civicrm/packages/HTML/QuickForm.php(2084): 
    PEAR_Error->__construct("unknown error", -1, 16, (Array:2), "Form rule 
    callback returned invalid value in HTML_QuickForm::validate()")
#3 /var/www/vhosts/<site>/httpdocs/wp- 
   content/plugins/civicrm/civicrm/packages/PEAR.php(575): 
   HTML_QuickForm_Error->__construct(-1, 16, (Array:2), "Form rule callback 
   returned invalid value in HTML_QuickForm::validate()")
#4 /var/www/vhosts/<site>/httpdocs/wp- 
   content/plugins/civicrm/civicrm/packages/PEAR.php(236): 
   PEAR::_raiseError(NULL, NULL, -1, 16, (Array:2), "Form rule callback 
   returned invalid value in HTML_QuickForm::validate()", 
   "HTML_QuickForm_Error", TRUE)
#5 /var/www/vhosts/<site>/httpdocs/wp-
   content/plugins/civicrm/civicrm/packages/HTML/QuickForm.php(1588): 
   PEAR::__callStatic("raiseError", (Array:7))
#6 /var/www/vhosts/<site>/httpdocs/wp- 
   content/plugins/civicrm/civicrm/CRM/Core/Form.php(555): HTML_QuickForm- 
   >validate()
#7 /var/www/vhosts/<site>/httpdocs/wp- 
  content/plugins/civicrm/civicrm/CRM/Core/QuickForm/Action/Upload.php(153): 
  CRM_Core_Form->validate()
#8 /var/www/vhosts/<site>/httpdocs/wp- 
  content/plugins/civicrm/civicrm/CRM/Core/QuickForm/Action/Upload.php(136): 
  CRM_Core_QuickForm_Action_Upload- 
  >realPerform(Object(CRM_Event_Form_Registration_Register), "upload")
#9 /var/www/vhosts/<site>/httpdocs/wp- 
content/plugins/civicrm/civicrm/packages/HTML/QuickForm/Controller.php(203): 
  CRM_Core_QuickForm_Action_Upload- 
  >perform(Object(CRM_Event_Form_Registration_Register), "upload")
#10 /var/www/vhosts/<site>/httpdocs/wp- 
  content/plugins/civicrm/civicrm/packages/HTML/QuickForm/Page.php(103): 
  HTML_QuickForm_Controller- 
  >handle(Object(CRM_Event_Form_Registration_Register), "upload")
#11 /var/www/vhosts/<site>/httpdocs/wp- 
  content/plugins/civicrm/civicrm/CRM/Core/Controller.php(351): 
  HTML_QuickForm_Page->handle("upload")
#12 /var/www/vhosts/<site>/httpdocs/wp- 
   content/plugins/civicrm/civicrm/CRM/Core/Invoke.php(309): 
   CRM_Core_Controller->run((Array:3), NULL)
#13 /var/www/vhosts/<site>/httpdocs/wp- 
   content/plugins/civicrm/civicrm/CRM/Core/Invoke.php(84): 
   CRM_Core_Invoke::runItem((Array:15))
#14 /var/www/vhosts/<site>/httpdocs/wp- 
   content/plugins/civicrm/civicrm/CRM/Core/Invoke.php(52): 
  CRM_Core_Invoke::_invoke((Array:3))
#15 /var/www/vhosts/<site>/httpdocs/wp- 
  content/plugins/civicrm/civicrm.php(1246): 
  CRM_Core_Invoke::invoke((Array:3))
#16 /var/www/vhosts/<site>/httpdocs/wp- 
  content/plugins/civicrm/includes/civicrm.basepage.php(141): 
  CiviCRM_For_WordPress->invoke()
#17 /var/www/vhosts/<site>/httpdocs/wp-includes/class-wp-hook.php(286): 
   CiviCRM_For_WordPress_Basepage->basepage_handler(Object(WP))
#18 /var/www/vhosts/<site>/httpdocs/wp-includes/class-wp-hook.php(310): 
   WP_Hook->apply_filters(Object(WP), (Array:1))
#19 /var/www/vhosts/<site>/httpdocs/wp-includes/plugin.php(515): WP_Hook- 
   >do_action((Array:1))
#20 /var/www/vhosts/<site>/httpdocs/wp-includes/class-wp.php(728): 
   do_action_ref_array("wp", (Array:1))
#21 /var/www/vhosts/<site>/httpdocs/wp-includes/functions.php(960): WP- 
   >main("")
#22 /var/www/vhosts/<site>/httpdocs/wp-blog-header.php(16): wp()
#23 /var/www/vhosts/<site>/httpdocs/index.php(17): 
 require("/var/www/vhosts/<site>/httpdocs/wp-blog-header.php")
#24 {main}

Any idea where to start?

I also have a problem with the payment section of the registration form not displaying sometimes, but not other times.