Thanks for reporting this. That error usually means the browser failed to load a route-specific JS chunk for the UI (in this case the Deployments page). After an upgrade, the most common causes are cache/proxy mismatches or an asset being served with the wrong status/MIME type. Quick checks to try: 1) Hard refresh the UI - Open DevTools (F12), keep it open, then right-click the reload button and choose “Empty Cache and Hard Reload”. - Alternatively, clear site data for your Prefect URL (Application tab → Clear storage → Clear site data). - If you see any service worker under Application → Service Workers, click Unregister. 2) Restart the server/container - If you’re running via Docker/Compose, restart the Prefect container so the UI assets served match 3.4.15. - Verify the version with:

prefect version

3) Inspect the failing asset request - In DevTools → Network, click the failing

Deployment-*.js

request. Note the Status code and Response headers. - You can also test from the host:

curl -I <http://localghosts:4200/assets/Deployment-D15xLX-O.js>

What do you see? - 404: Usually indicates a cache mismatch (old index.html referencing a chunk that no longer exists) or a reverse-proxy rewrite that breaks /assets. - 200 but Content-Type text/html: A proxy is likely rewriting the path and serving HTML instead of JS. - 401/403/CORS or Mixed Content: A proxy/auth/cert issue. 4) If you’re behind a reverse proxy or a subpath - Ensure /assets/* is forwarded directly to the Prefect web server and is not cached or rewritten. - Disable proxy caching for the UI assets during troubleshooting. Example Nginx stanza:

location /assets/ {
  proxy_pass <http://prefect:4200/assets/;>
  proxy_set_header Host $host;
  proxy_cache_bypass 1;
  add_header Cache-Control "no-store";
}

A couple of clarifications that will help me pinpoint this quickly: - How are you running self-hosted? Docker/Compose,

prefect server start

, or something else? - Are you serving behind a reverse proxy or at a subpath? - Do other UI pages load, and is it only the Deployments page that fails? - In the Network tab, what’s the exact status and content-type for that

Deployment-*.js

request? - Is the host name “localghosts” intentional, or is that a typo for “localhost”? For reference, self-hosted docs: Self-hosted Prefect 3 Releases: Prefect releases If you can share the response details for that asset and your deployment/proxy setup, I can give you exact next steps.