Where is the Cinnamon LookingGlass log?

It's in

~/.xsession-errors

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Prior to Cinnamon 3.8.8 it was

~/.cinnamon/glass.log

In cinnamon 3.8.x the glass.log was combined into the ~/.xsession-errors file.

You can view and filter only (looking)glass.log entries on the Log tab of the Melange-Cinnamon Debugger application. To open Melange, Right click on the task bar in cinnamon and then select Troubleshoot -> Looking Glass.