Sorry for the late response, I couldn't wrap my head around the issue, but I found it. I was actually right, but I didn't notice that on the General Options -> Sources 2 the product categories were selected to be returned as results.
Basically if anything besides products or variable products is passed to the results page, it will cause the error message to appear, it looks like.
I'm going to have to implement some kind of filter to prevent passing other objects to the results page, if woocommerce is active and the destination is the woocommerce product results page.
I've left the override active and deselected the product categories. If you wish to search them, don't forget to turn off the override options. I'm going to come up with a solution for the upcoming release, so after the next update it should work correctly.Best,
If you like my products, don't forget to rate them on codecanyon :)