I have a SQL 2005 instance with two SQL Agent backup jobs. The full runs at 1:00 am and the tran log runs every 15 minutes.
The tran log backups ran perfectly until the nightly full backup. Then, the job ran once more. It failed after that with the error - 'BACKUP LOG cannot be performed because there is no current database backup.'
If SQL thought the last full backup wasn't valid, why did it run the tran log backup job once after the full backup?
Answered my own question. I do a full backup at 3:00 am - it takes about 42 minutes. I do a DBCC Shrinkfile on the log file at 3:00 am. There's one log backup between the full backup and the DBCC shrink file. The DBCC Shrinkfile script puts the DB in SIMPLE Recovery mode. Even though it switches it back to FULL, there has to be a full backup before the tran log backups will work.
The solution - run the DBCC Shrinkfile before the full backup.
Why are you shrinking the log file? It should never be done automatically. And I completely disagree with switching the recovery model to SIMPLE as that breaks the log chain and limits your data recovery options.