Shopping cart doesnt update and buttons in administration frozen

Hi,

Afte 7 years my previous customer asked to have a look at their website. Due to the topic and products they really need to continue using a theme like ForChildren.
I managed to change their shop from Magento to Woocommerce, but run into some issues with the (old) ForChildren theme.

The most important issue is that once you add a product into the shopping cart, you can't update it anymore. Wether you increase the amount of products or try to remove it from your basket, the waiting-icon sits there spining forever. I tried several online solutions but none work. Disturbing is that the problem does not occur with the default Woocommerce Storefront theme which leads me to believe it's with the ForChildren theme. Any ideas?

Second issue; in the administration area when I try to open the ForChildren settings, none of the upload or save buttons work. I can't change sliders or logo locations etc. So I ended up overwriting current images on the same locations. Why is this happening?

Am I running into compatibility issues with 5.8.2. wordpress? And if so, how do I fix them without having to change the whole theme?

? :(