Incremental SQL Backups drop to zero bytes after running once or twice

[vanillarex]

We are having problems backing up a MS SQL Server with 20 SQL Instances.
Full backup works without a problem, the day after full backup too, but the second or third incremental backup after full backup will only backup 0 bytes. This will continue until the next full backup.
The backups are shown as successful, so there is no error message with this.
This happens when we are backing up all instances in one job as well as when we were backing each instance for itself.

[pkh]

Check that your log files are big enough to accommodate the changes to your databases.