Transaction log reported full but is not

Batch jobs error emails report: The transaction log for database 'WebDB' is full.

No, it isn't. Its 90% free. The transaction log backups run normally. My hardware guy tells me the hardware is running perfectly, with plenty of space. SQL Server 2008 SP3, but not R2.

I'm a developer, we no longer have a DBA. So it falls to me to try and fix this.