Cumulus 1 creates diagnostic logs in its diags folder; diags is a subfolder of the folder where the Cumulus executable is stored. It contains 10 files named cumulus.~og and cumulus.001 to cumulus.009. These represent logs for previous 9 Cumulus restarts and the current log reporting what has happened since the last time Cumulus was started. These files log many of the actions performed by Cumulus 1. They will show the Cumulus specific files that have been accessed, values found or calculated from log files, before and after values for updates to highs and lows for monthly and annual extreme records, rejected spikes, and much more.
Note re Cumulus MX
Please see MX Diagnostics article
Solving Reported Errors
The diagnostic files also log error messages, often with extra information compared to the error window (accessed by clicking error light/indicator), and may help you if you have a problem.
When you ask for support via the forum, you often need to upload (as a zip) one or all of the diagnostic files especially for problems that occur when you start or restart Cumulus. In Windows Explorer, open the Cumulus installation folder (default is C:\Cumulus). Right-click on the 'Diags' folder, and from the pop-up menu, select 'Send to -> Compressed (zipped) Folder' (available in most Windows versions). This will create a file called Diags.zip. Post your message in the forum, and attach the file using the 'Upload attachment' section below the area where you have typed your message (browse for the file, then click 'Add the file').
See Debug.log as this is separate file stored in the top directory.