@jakeoh : can you activate console in your browser.
If you use Firefox : see the configuration
If you 500 error at right : click on the little triangle at left
Send the screenshot on our mantis.
Better
1. Activate debug mode: manual.limesurvey.org/Debug_mode
2. Try to "Ouvrir l'URL dans un nouvel onglet" (you say you read freench
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3. If the error is same (unsure : sometimes need POST request) : send the HTML file on mantis.