The Console Log
Tunnelblick sometimes outputs detailed diagnostic information to the macOS logs.
To view the logs, launch the Console application, located in /Applications/Utilities.
If you don't see a list of different logs to view on the left side of the Console window, click "View", then "Show Log List".
If you don't see a toolbar at the top of the Console window, click "View", then "Show Toolbar".
Proceed as follows for your version of macOS:
There is a "Filter" (or search) text box in the upper right corner of the Console window. If you type "tunnelblick" (without the quote marks) into the text box, Console will only show messages from Tunnelblick, not from other programs.