Sql Server log: is 'Backup|Log was backed up' the same as 'BackupDatabase: Log files done'?
The final reason for this effect was that the Backupexec process responsible to take regular log backups was still present and running (so IT monitoring did not alert anything), but it was kind of internally crashed, so it did not do it's job properly.
In order to address this kind of event I decided to add the log file events and it's metadata details to the SQL Server monitoring. Once every day those information is captured and added to the monitoring information repository, where I can run reports and queries against to see progress and find out abnormal behavior.
This ended in a report about the log files looking similar to this sample. It shows the file size over time of each TL and also the percentage of usage at each point. If point size increases, the file contents grow. If they grow too much, the file size increases. Dramatically file size decrease was caused by manual shrink operations (followed by rolling eyes after it grew and grew again...). Basically log file usage and backups should be aligned in a way it almost never grows.

This is very useful to keep an eye on and eventually could also be used to alert by using this report as a data driven subscription (which I have not tried yet).