Error 500 when submitting to adopt a Hewan (pet)

This is the page in question: https://www.hewan.org/forum/adopt.php

What the error looks like: https://i.imgur.com/JQ71f4l.gif

And the error message:

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<title>Error 500</title>
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<font size=4 face="arial">Error 500 :(<br><br></font>
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<span style="font-size:8pt; font-family:arial; color:#000000;">
<p align="justify">An error occurred.</p>
<p align="justify">If the problem persists, probably the application you use does not work correctly, because someone is working on it, or because there is a problem in the application. In the latter case <a href="http://en.altervista.org/feedback.php?who=feedback&lang=en" target="_blank">contact us</a>.</p>
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<p><a href="javascript:history.back()">Go back</a></p>
</center>
</span>
</body>
</html>