Missing orders in Woocommerce backend

I have several orders that although executed properly, do not show up in my orders lists and can thus not be handled.
I discovered these missing orders while supporting clients at a later stage whose order I could not find and refer too.
After investigation, I can ascertain that:

1.
The orders were completed with payment (by payment gateway) received and dispatch of serial number (we sell software) arrived in the client email. The invoice has also been issued. The order and serial number are visible in the client’s front end account page.

2.
The client is registered in the user database as a customer.

3.
The only problematic place is woocommerce back-office. The log shows an error that I can trace to the problematic orders (error is present 7 times for 7 missing orders): (see private content for error message)

4.
Other orders have been received properly after these errors. Previously we completed around 3000 orders in the same way on the same server.

5.
After reaching to woo-commerce support, they tell me the problem is originating in my theme – Enfold – that overrides a necessary resource of woocommerce.

I do not know how to go further as they tell me my theme would certainly need an update to solve this. I run the latest version now, not the case when the error occurred.

Thanks for your help