SQL transaction logs are automatically truncated with a full backup. Doing a tlog-only back up means you're doing a differential backup. Pros and cons to that: with differential, your backup media storage is cheaper, as well as backup time, but recovery in case of a failure is horrific in terms of complexity. A daily full backup suits most SDE installations, as well a pulling a full before significant data changes. Ask you're whomever is maintaining your SQL to pull a full back up in either case, and see if that error goes away.