The solution for this problem was rather strange. I hired a developer who helped me solve this:
- increased memory limit in
config.php(not sure if this was causing the problem since it didn’t look like a time-out or anything) - in the file
regenerate-thumbnail.phpthe linewp_update_attachment_metadata( $image->ID, $metadata );was commented. I’m sure I did not do this myself, but I tried a lot of plugins to solve the problem; maybe one commented this?
To further enhance the performance, it’s possible to generate thumbnails twice, e.g. for 1X and 2X displays. I’m not completely sure, but as far as I understand, in my case this will lead to generation of the following thumbails:
- Catalog Images: 247x247px AND 494x494px
- Single Product Image: 510x510px AND 1020x1020px
- Product Thumbnails: 130x130px AND 260x260px
Your browser then will use srcset to check which files are available and by that choosing the best variant to display.
Hope this helps someone else as well.