I'm not sure whether it is related or not, here is my experience with issue.
By default, bagisto set Weight as system-defined attribute with type Text.
If you set is_required to False and put empty text in Weight, it will cause error when doing Add-To-Cart.
You can test it by intentionally setting different product weigh to empty and try Add-To-Cart.
For me, only one product is causing that issue and I fix it by setting Weight to 0.
Easy fix, and 4 hours well spent (not really).