I have this bug without change anything, what I can do now? How I can fix it?
Btw, respect, you made the best theme for MyBB

I think you've removed a symbol from the code. Check the last thing you modified. If you changed the language, double-check what you changed and go step by step.

I get the exact same bug from a fresh install of Curves UI.

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Fixed it by changing the language file in ACP.

Languages>English>new thread.lang.php

Changed

thread_subect text to "Subject"

It was "Title <span class="text-danger" style="font-size: 10px; font-weight: 700">REQUIRED</span>"

I'm not sure if that was left over from another them I'd installed or not. I did install the language files via php before importing Curves. I dunno. Anyway, it worked.