Hello Thomas Blair,
Thanks for your question.
I will recommend you run the Diagnostic Interface to further troubleshoot.
If you have Application Insights configured, check the Failed Requests and Exceptions in Application Insights, you could also download the code from the Azure Web App and run it locally in your development environment to reproduce the issue.
See similar issue: https://stackoverflow.com/questions/36967675/how-to-resolve-error-500-on-azure-web-app
Regards,
You can mark it 'Accept Answer' if this helped