Hi lucn,
I’m sorry to hear that you’re experiencing that frustration. But, unfortunately, I can’t think of a reason that could explain that specific issue.
While you are on the Theme page, I suggest that you open Developer Tools in your browser and look at the Console for any errors that may provide a clue.
Also, try accessing the Themes page using a different web browser to see if it works as expected. If it does, it could indicate an issue with a plugin/ extension in your regular browser.
I’ll leave this open, and if you learn anything new, please feel to share.