“Sold out” error message when registering event with PayPal Express payment (event is not full though)

Reproduced here: https://www.evergreenmtb.org/education/adult-co-ed-classes

Payment Processor: PayPal Pro
User fills out event registration info, clicks the PayPal button, logs into PayPal and submits the payment, back on the confirmation page clicks on Continue.

The result: The user gets the following error messages (which are incorrect as the event is not sold out):
{{
Unfortunately some of your options have now sold out for one or more participants. You have been returned to the start of the registration process and any sold out events have been removed from your selections. You will not be able to continue until you review your booking and select different events if you wish. Sold out: Please note that the options which are marked or selected are sold out for participant being viewed.}}