Hello Tomas -
We are aware of similar reports where the gallery image is not being properly generated on eBay once the listing has been submit and the Header image is actually showing correctly. Our engineers are looking into some of the feedback our community reports as having received by eBay, but as noted earlier in this thread, as long as your Header image is showing in the listing, we have been able to successfully submit your image to eBay.
As for your mention of canceling the affected listings, I suggest that you first try revising them directly on eBay; if the eBay tool for generating the gallery image is failing. You should be able to go to the Revise Your Item form for the listings and then remove and replace the Header image (you may need to replace all of the listing images when revising) on your listings. This should then prompt eBay to make new attempts at generating the gallery image.