This type of error happens when active quote is requested by id and either id is missing from request or quote is already inactive. Common case is that extension providers try to laod the quote object with getActive($quoteId) method in after order placement events without thinking that if the requested quote is not found Magento2 will throw a exception that bubbles up to: 

Placing an order with quote_id xxxx is failed: No such entity with cartId = xxxx

Most common conflict that affects OneStepCheckout is found in following extension  mageplaza/module-smtp . Error happens in vendor/mageplaza/module-smtp/Plugin/AccountManagement.php where plugin tries to load a quote that is already set to be inactive by payment method.

To solve this issue:

  • either change the quote request to count also inactive quotes
  • or move the load call to try/catch block inside the same method (handling the save but I suspect that this plugin will always fail to do what it needs to do in this case) 
  • or add to the first if clause to verify if quote is active