Incorrect operation of the Bitrix ordering component?

Question

1C-Bitrix

Hello!
Online store. As you know, the sale.order.ajax component has changed and is now very difficult to customize. Or rather, his pattern.
So, to solve this problem, I found an old template and installed it by connecting "compatibility with previous templates" in the settings.
Everything generally works fine, except for one thing - order confirmation.
And it works like this: if the user is not registered, he enters his information in the fields and everything is sent.
But if it is registered, then when you click on "place an order" either all the fields disappear and one button remains, or in addition the error "Fatal error: Unknown: Cannot use output buffering in output buffering display handlers in Unknown on line 0".
The most interesting! while the order is still being shipped! but! sent partially, namely - the first product of all. And only one.
I understand that it could not be without mistakes. But can you suggest something?
Thank you.