[JDHill]

I'm not too sure if I understand which hour is used in the "Location and Time" Panel.
From the help file, it looks like it's Greenwitch Mean Time, so one needs to convert it to local time, right ?
Anyway, the "PM" does not seem to appear anymore, making it quite confusing to set the proper time.
Wouldn't it be better to use the 24h format ?

No, it is specified in local time, with GMT compensation being done automatically by the plugin based on your currently-chosen latitude & longitude. Regarding the display style, you can customize that however you like by right-clicking on the date/time picker and clicking Customize Format.

I wasn't using the 2.5.1 Maxwell but the 2.5.0, and the Maxwell plugin was 2.5.12
Yet, it would be nice to have a specific error message in this particular case.

I don't disagree, but this is not something the plugin can change.