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Is there a specific log file for this installation? Where can I find it?
For the installation, there is a log, but I am asking for the log weewx writes every archive_interval. Mentioning the archive_interval again: consider increasing it. Rendering two or more skins will probably take longer than the archive_interval, from time to time.
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General question about the "livegauge" and the reports, how I change the "m/s" by "km/s" for the wind speed ?
Yes, it is also described in the about.html:
[StdReport]
[[Bootstrap]]
[[[Units]]]
[[[[Groups]]]]
group_speed = km_per_hourOr in
[StdReport]
[[Defaults]]
... for all skins (due to a bug, this doesn't work with versions < 4.1). If you only do it in the skin's "fr" section, the other languages will still use the unit defined with the unit_system in each lang/xx.conf