Hey Polish,
I've checked and found that you have restore payplans to version 3.7.1 again.
I've checked and found that you are using Payu Poland payment method at your site , which is deprecated in PP 4 , because we are unable to get merchant sandbox account. That's why it giving 500 error.
If you don't want to use this payment method further then disable this plugin and do the upgrade.
It's mentioned in transition doc also.
Are there any apps or payment methods deprecated in PayPlans 4?
Yes and they are properly listed above. If you are using any of them at your site, we strongly advice against upgrading to 4.0. What you can do for now is to remain on version 3.7, but at the same time, do provide us the sandbox merchant accounts for the payment methods, so we can add those payment methods back. Once all that is done, you may then perform the upgrade.
Update me if you have any query.