> For the complete documentation index, see [llms.txt](https://docs.defguard.net/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.defguard.net/support-1/troubleshooting-guides/core.md).

# Core

- [h2 protocol error from reverse proxy](https://docs.defguard.net/support-1/troubleshooting-guides/core/h2-protocol-error-from-reverse-proxy.md)
- [Unable to sign in with correct credentials](https://docs.defguard.net/support-1/troubleshooting-guides/core/unable-to-sign-in-with-correct-credentials.md)
- [User lost access to their 2FA methods](https://docs.defguard.net/support-1/troubleshooting-guides/core/user-lost-access-to-their-2fa-methods.md)
- [Enrollment URL Changed](https://docs.defguard.net/support-1/troubleshooting-guides/core/enrollment-url-changed.md)
- [Failed to parse duration](https://docs.defguard.net/support-1/troubleshooting-guides/core/failed-to-parse-duration.md)
- [Core cannot connect to Gateway or Edge after restart on single-host deployments](https://docs.defguard.net/support-1/troubleshooting-guides/core/core-cannot-connect-to-gateway-or-edge-after-restart-on-single-host-deployments.md)