Hi @arkano, sorry, I was not able to reproduce this error.
On a clean installation of WordPress 5.8 with Font Awesome plugin 4.0.1 and WooCommerce 5.5.2, I created a new virtual download product, clicked “Add product gallery images” from the Product Gallery sidebar widget, uploaded images using the modal window it popped up, saved and added to them to the product.
I was also able to click the “Add Font Awesome” button on the “Product short description” editor to pop up the Icon Chooser modal window to select and insert an icon shortcode.
After saving the product, I viewed the result front end product page and verified that the product gallery images and the icon were all displayed as expected.
If you (or others stumbling onto this thread) see the same error behavior, please note any other details you can to help reproduce the error (screenshots might be helpful too).
I have fixed some other issues (that may or may not be related to the error you experienced) in a forthcoming release. If you’d like to test the pre-release, you could download the installable zip file from our GitHub repo here.