I had similar issue yesterday where I wrote about it (https://forum.userstyles.org/discussion/66867/error-404-on-my-style-page#latest
). And it is happening again. Thankfully I know how to fix it, But what I am complaining about is if you can fix it so I don't have to upload my thumbnail every day to resolve the bug.
I attached images of how bug where it gives error alert then thumbnail is gone and it doesn't recognize that I installed that theme. If I go to edit section it doesn't show the thumbnail, it shows broken image icon. Please fix it.