Hi Karsten,
If the site URL was recently changed, then make sure to have it corrected on the back-end settings, as well as in the wp-config.php file. I highly recommend this step, as this problem might re-occur in the future with other plugins as well, requesting the thumbnail images. The double slash is very likely related to that as well.
Additionally, you can use this custom code in the functions.php file in your active theme directory to force the correction in the image files in the results (copy from line 3):
PS: be careful when adding custom codes, make sure to have a full site back-up first, so you can restore it in case of issues!
Best,Ernest Marcinko
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