If you have to prove that a backup was made, you can mark a synchronisation for a Backup proof – the tick is on the Options tab. Unlike the log, whose extent depends on the level set and which is cut after a few days, the proof is meant to be kept.
The directory is set in the logging settings under Path for backup proofs. In it AutoFileSync creates one file per synchronisation, named after it, in JSON format:
<proof directory>\<name of the synchronisation>.json
The field names are written in the language the program shows at the time of the run; a file begun in one language is continued under its list name.
A run that had nothing to do writes no entry. With a pause of a few seconds there would otherwise be hundreds of meaningless lines per hour, among which the runs with real content could no longer be found.
The error messages in the proof come directly from the synchronisation, not from the log. The log level set therefore has no influence on what the proof contains.
If an existing proof file cannot be read, it is not replaced but left alone: it is evidence, and the content already there weighs more than the entry that cannot be added right now.