Steps to reproduce:

1. Create a new project and use IN as the project key.
2. Go to https://<instance name>/projects/IN/issues
   (This example we use IN, there are a list of reserved words on the comment below)

Expected Behavior:

• JIRA should display issues of IN project

Current Behavior:

• JIRA doesn't display any issues

Checking from the network request /rest/issueNav/1/issueTable, it returns 404 Bad Request with the below response:

{"errorMessages":["Error in JQL Query: Expecting either a value, list or function but got 'IN'. You must surround 'IN' in quotation marks to use it as a value. (line 1, character 11)"],"errors":{}}

NOTE: it only affects the Project Navigation's issue panel, reverting to the old project view will workaround this issue at the mean time.

Workaround
Change the Project Key of your project. Make sure it avoids one of the Reserved JQL words (see this comment below).