An LTI link in Moodle gives the error message "An error occurred when launching the external tool: Debug error: Invalid signature"

This error usually occurs because the consumer key and secret are wrong. In many cases, a browser's password manager may be filling in the secret automatically with the logged-in user's Moodle password, rather than using the secret that the Moodle administrator configured. 

To fix this issue, follow the steps below to edit the LTI link. 

1. Toggle into editing mode. Click Edit and select Edit Settings.
   
   
2. On the Tool Settings page, click Show more...
   
   

3. Confirm that Preconfigured tool is set to Automatic, based on launch URL. Make sure the Shared secret field is blank⁠—this step will ensure that the password the Moodle administrator configured for the tool is used.
   

   
   

4. Click Save and return to course to save these settings.