I believe you need to format the message and remove the GUID. So all the messages with GUID will be aggregate to single message.
You can use regex to format the messages and remove the GUID. The sample query look like this and use as needed.
The sample error message is like this
Error occurred. Exception: System.Exception: my custom error message: 1121fd05-065b-499f-b174-2a13efdaf8b5
And the Sumologic query
_sourceCategory="dev/test-app"
and "[Error]"
and "Error occurred"
// | timeslice 1d
| formatDate(_receiptTime, "yyyy-MM-dd") as date
| replace(_raw,/my custom error message: ([0-9A-Fa-f\-]{36})/,"my custom error message") as finalMessage
| count date, finalMessage
| transpose row date column finalMessage
This video shows step by step guidance. https://youtu.be/Nxzp7G-rUh8
