Web server, reverse proxy and TYPO3 middleware
Check site base and fixed media URLs in Fluid, TypoScript, and content. During proxy operation, additionally examine the detected public HTTPS connection.
Headers and HTTPS behavior can occur before the TYPO3 application. Compare the publicly delivered response to the intended hosting configuration before attempting a sitepackage change.
Where to check the setting in TYPO3
Separate TYPO3 middleware, web server and reverse proxy as possible header sources. Mixed content also checks site base, stored external resources, and fluid output. A new header rule in the application can be overwritten by the proxy.
Example of this specific finding
Illustrative control case: An HTTPS page integrates a script via http://. Browsers can block this insecure resource; a form may not work then.
follow-up audit in TYPO3
Test HTML, downloads and static resources separately. A blanket header rule can capture different answers and block desired embeddings.
Switch the responsible integration to an actually functioning HTTPS resource and re-check the browser console and the affected function.
Derogations and scope
Simply replacing the http string with https does not help if the target does not offer a valid HTTPS.
Reference: TYPO3 13.4 with EXT:seo. For other major versions and own extensions, responsibilities may differ; check the actually installed output.
Classify the general error
Definition, system-independent causes and further correction examples: Track the causal involvement to the source