JabRef version

Other (please describe below)
6.0-alpha.403

Operating system

GNU / Linux

Details on version and operating system

ubuntu:22.04

Checked with the latest development build (copy version output from About dialog)

• I made a backup of my libraries before testing the latest development version.
• I have tested the latest development version and the problem persists

Steps to reproduce the behaviour

1. Start JabRef.

2. Open File → Preferences → HTTP Server.

3. Enable the HTTP server and apply/confirm with OK.

4. Run the following command (adjust host/port to match the UI if needed):
   curl -sS --max-time 5 -D - "http://127.0.0.1:23119/libraries/demo" -o /tmp/demo.json

5. (Optional) Check the file size:
   wc -c /tmp/demo.json

Expected Behavior

• The endpoint returns HTTP 200 OK.
• The response Content-Type is application/json.
• /tmp/demo.json contains the demo JSON payload (baseline observed ~10,975 bytes).

Actual Behavior

• The endpoint returns HTTP 500 Request failed.
• The response Content-Type is text/html; charset=ISO-8859-1.
• /tmp/demo.json is much smaller (observed ~1,031 bytes), indicating an error HTML payload instead of JSON.

Appendix

• Reproducible after merging Improve http server code #14489 , namely Commit: 636b37c
• After restarting JabRef, this issue still exists.

Screenshot before

Screenshot after