In this case I'm a bit clueless as I certainly did not change this setting in the Backup Job, it was always set to LAN Target and I would like to keep it that way.
I exported the logs for the affected backup and when I search for "StgBlockSize" it clearly states that 512Kb is used <StgBlockSize>KbBlockSize512</StgBlockSize>. I checked for the last 6 days and it always stated KbBlockSize512. Both the Backup Job and the Backup Copy Job actually did run successfully this morning (apparently with a block size of 512Kb).
If this is the case, the Backup Copy Job must have changed, for whatever reason. I also exported the logs there and it was always set to "Automatic" for the last 6 days. Why does it suddenly decide to use 1024 Kb then?
Anyway, there is no "Local target" in the backup copy job settings, only "Auto" (which was always set), "None", "Dedupe-friendly", "Optimal", "High", "Extreme". Which one of these corresponds to 512 Kb?