Hello Tomasz,
I’m using 0.35.3, i had that behavior after setup an author image in webp.
The problem was available on all website with different theme.
I fixed it removing the image, setup a new image.
Then switch them for all website then regenerate the thumbmail and switch again to old theme and works.
Let me know if you want any info or if i can close the post.
Best Regards,
Davide Franchioni