Shopping cart loading stuck problem

When I’m on mobile and click on the checkout page “Payment via PayPal” and go to the PayPal page and then go back to the site, then the spinner of the charge gets stuck and the whole site gets stuck

This is the stuck object:
vi-wcaio-sidebar-cart-loading-wrap

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In addition, sometimes I get an error in the “Send Order” button. It writes instead of “Send Order” -> “undefind”

What is the reason for this?
* Important to mention, it happens occasionally. Sometimes it works fine and sometimes I get the “undefind” button.