Summary
Following error pops up when trying to access an open ticket as a customer

Error Code: 500 Internal Server Error. The request was rejected by the HTTP filter. Contact the server administrator (12217)

Applies To
MachPanel all versions
ISA Server

Solution
On investigation it was found that the problem occurs upon accessing the URL in an environment controlled by ISA server. The ISA HTTP filter blocks the connection causing the error.

To solve this problem please do the following on ISA Server:

1. Edit the publishing rule.
2. Go to traffic tab
3. Click the filtering button
4. Select configure HTTP and then un-check Verify Normalisation.

Once the configuration is applied everything will work as expected.