Hello Holger,
could you already clarify your question? Do you mean the transaction SM21? Which severity has the syslog message "transaction canceled 14 027"? In the syslog you cannot decide, which syslog message should be stored in the syslog, you can only filter the syslog messages. There is no possibility to suppress the syslog messages.
As far as I see in our test system in SE92 the syslog message "transaction canceled" has the syslog number D01 and has the severity "red with alert". This is why you see it with the selection "Only Messages with high importance". You could change the severity of the syslog message in SE92.
Regards, Alwina