since 6.4.6.4 #_BOOKINGBUTTON” not canceling events

Till version 6.4.6.3 my $EM_Event->output(“#_BOOKINGBUTTON”) produced a button without errors.

Since 6.4.6.4 a logged in user can’t cancel an event.

In console i see an error:
[Error] Failed to load resource: the server responded with a status of 400 () (admin-ajax.php, line 0)

so I went back to 6.4.6.3

The page I need help with: [log in to see the link]