Hello Lucio,

I have checked your site and nothing found that will cause the issue.
Only one point, payment app instance is missing , so when user try to payment it will through 500 error. Please create payment gateway instance.

Please Next time if you will get this issue then please update me before disabling anything on site.

At any time, error 500 appears when PayPal is active. If I correct the payment method, does it mean that I have to acquire a payment method? Can you help me with that please, I just activated it again, hoping it's already working. I hope you can support me

Hello Lucio,

I checked again your site. you have 2 plans on site , one is free and another one is paid.
In case of free plans no payment method is required but for paid plans payment method is required, you haven't install any payment gateway app, that's why 500 error giving on invoice checkout page. see attached screen shot.

I have setup the offline payment app at your site and now no 500 error will not shown to user, it's working fine.
see attached screen shot.

Please setup the payment method from which you want to collect payment like paypal or any other method and check this issue again. Update me if you have any confusion.