There are a few reasons why this might be occurring. Please follow the steps below to fix this error.
1. Authentication key not cleared properly
The authentication key that Nintex Live issues to facilitate secure communication is stored in memory in both the IIS worker process and the SharePoint timer job. Sometimes the authentication key doesn't get cleared properly when changing the Nintex Live settings in Central Admin, and it can get stuck in an invalid state.
Please try resetting IIS on all SharePoint web servers and also restarting the timer job. This will cause Nintex Live to get a new authentication token.
2. The internal network is blocking communication
Nintex Live is essentially an external website that an on-premises SharePoint farm communicates with. It is common for corporations to use firewalls and proxies to restrict access to external websites for security purposes. For Nintex Live to work correctly, it needs to be added to the 'safe list' of allowed websites, or else the internal network will block communication.
Please make sure that:
https://liveservices.nintex.com
AND
http://nintexlive.blob.core.windows.net
are added to the safe list.
3. Software Assurance not part of your licence.
Nintex Live access is only made available to customers who have a current Software Assurance with Nintex. Please contact admin@nintex.com to discuss Software Assurance.
4. It is possibly a bug and Nintex Support will need to investigate further
If there is a bug or other condition stopping the Nintex Live Catalog from showing, the exception should be visible in the SharePoint ULS logs. Please contact Support@nintex.com and provide the logs from around the time you tried to access the catalog.
If there isn't any information in the ULS logs under the "Nintex Live Communication" or "Nintex Live Provider" category for that time, please ensure that the Nintex Live diagnostic logging is turned on.
To do this:
1. Navigate to Central Administration.
2. Click on Monitoring and then Configure diagnostic logging.
3. Enable the "Nintex Live" checkbox and set the “Least Critical Event” to report as Medium.
4. Once the logs are collected, the log level can be reset to default.
5. Please zip the logs and email to Nintex Support.