Paypal can't be activated as subscription payment method for Quicken Classic (Mac) subscription

When attempting to update the payment method for my annual Quicken Classic (Mac) subscription, I access the "Quicken: My Account" panel (by clicking on the Q icon in the application title bar), then select "Payments". I then click the "Update payment" link next to the current default payment method (which is a credit card). The "Set Up Payment" window appears, and it includes two tabs: "Card" and "PayPal". I select the "PayPal" tab, and then click the yellow "Pay with PayPal" button. This launches a PayPal authentication dialog where I enter my PayPal username and password. After entering the PayPal username and password, there is a "captcha" presented that requires the user to either move a slider to a certain position or to enter a sequence of numbers correctly. This is where the process fails - either approach to the "captcha" fails to advance the authentication process (a correct slider position does not work, nor does entering a correct sequence of numbers). Performing the captcha correctly simply results in the captcha reappearing immediately afterward, in a permanent loop.

[Edited-Readability]