@hcabrera Thank you for that. I also use wordfence and your method solved my problem. Thank you so much.
@vanyukov It would be really nice if users are allowed to skip problematic images. Smushit can then store those images in a different section called skipped images or something similar. Then the user can try smush the images separately from there. It would help pinpoint the problematic images while also help decrease the probability of not being able to smush any further because of one problematic image.