There is missing a file - we have the same error on two installations. The error is in the popup:
"Backend" -> "Components" -> "PayPlans" -> "Subscriptions" -> "{Choose a subscription}" -> "Cancel Subscription". The following error message will be in the popup.
Warning: include(/public_html/administrator/components/com_payplans/themes/default/subscription/dialogs/cancel.subscription.php): failed to open stream: No such file or directory in /public_html/administrator/components/com_payplans/includes/themes/themes.php on line 170
Payplans: v. 4.0.8 (This is the first installed version on the site)