There are probably a good number of reasons this error shows up in your SQL Server log, along with the following message:
Message: BACKUP failed to complete the command BACKUP LOG DATABASE_NAME. Check the backup application log for detailed messages.
In my case, the recovery model was set to SIMPLE and the DB maintenance plan was set to backup all user databases, which included all databases set to SIMPLE. I have my suspicions as to why those production databases were set to SIMPLE but FULL is best in most cases anyway, so I set all recovery models to FULL, and manually backed up all databases. To see a list of all of your databases and their recovery models, run the following query:
select [name],databasepropertyex([name],'Recovery') as [Recovery]
order by [name]
Note that the following system databases are set to SIMPLE by default: master, tempdb and msdb.