Summary

List of gadgets went missing when trying to add one to the dashboard

Environment

JIRA 6.3.13
Korean Language Pack

Steps to Reproduce

It's not easily reproducible, but from one of the Support case:

1. Start JIRA 6.3.13
2. Install Korean Language pack from here
3. Change System Language to Korean
4. Create new Dashboard
5. Try to add new Gadget into the dashboard

Expected Results

Gadget list should be available as usual

Actual Results

Gadget windows is empty, and this message is found in the log:

2014-09-28 09:15:11,300 http-bio-8080-exec-36 WARN user 555x182356x1 1qfgr4p xxx.xxx.xxx.xxx /rest/config/1.0/directory.json [directory.internal.impl.DirectoryImpl] Could not retrieve directory entries from application-provided gadget specs: plugin gadget 'Key{pluginKey='com.pyxis.greenhopper.jira', location='gadgets/days-remaining.xml'}' failed validation
-------------------------------------------------------------------
2014-10-08 10:26:27,673 http-bio-8080-exec-2 WARN user 626x177724x1 1tmk3wu xxx.xxx.xxx.xxx,127.0.0.1 /rest/config/1.0/directory.json [directory.internal.impl.DirectoryImpl] Could not retrieve directory entries from application-provided gadget specs: null

Notes

Please try enabling Safe Mode in the Add-on Manager to verify if any plugins are doing this - if the problem goes away after enabling Safe Mode you can try enabling each plugin one-by-one, and testing the gadget list, to identify which one is at fault.
Also, certain language packs containing invalid characters are known to cause this problem to occur - this is an active bug tracked in TRANS-1233. Disabling the language packs, or editing them such that invalid characters are not containing in them could effectively workaround the problem.