By design, the last-chance threshold allows enough free log space to record a dump transaction command. There may not be enough room to record additional user transactions against the database.
Space available in the log segment has fallen critically low in database ’mydb’. All future modifications to this database will be suspended until the log is successfully dumped and space becomes available.
You can now execute only commands that are not recorded in the transaction log (select or readtext) and commands that might be necessary to free additional log space (dump transaction, dump database, and alter database).