JIRA Forbidden (403) page is garbled when using Agile Cards plugin and GZIP compression enabled

Summary

When trying to export more than 1000 issues to PDF (functionality came from Agile Cards plugin), JIRA's Forbidden (403) page is garbled. Only happened when GZIP compression is enabled

Environment

Tested on JIRA 6.2.7, 6.3.15, with Agile Cards 2.3.16

Steps to Reproduce

1. Install Agile Cards
2. Enable GZIP Compression in General Configuration page
3. Create more than 1000 issues
4. From Issue Navigator, use a JQL that will return more than 1000 issues
5. Click the Export > Agile Cards (PDF) button

Expected Results

Forbidden (403) page will be shown, as it has exceed the 1000 default issue export limit in JIRA

Actual Results

Garbled message is presented

Notes

• This only happened when GZIP compression is enabled
• Page is shown correctly when GZIP compression is disabled

Questions

What is causing JIRA GZIP compression to convert the page into garbled character?