Text and context

[MarcoP]

I don’t understand why people can be allergic to bringing context to the conundrum.

Who are these people? If you are not specific, we don't have a chance to help you with the things that you don't understand.

The specific context I point to is the 1450s 

The manuscript has been carbon dated to 1404-1438. You are not allowed to view links. Register or Login to view. let us know that a medieval costumes expert (You are not allowed to view links. Register or Login to view. from the Morgan Library) dated the costumes of Virgo and Sagittarius to the 1420’s.

You are not allowed to view links. Register or Login to view. published extensive research on the costumes that also confirmed the 1400-1430 period.

Something as late as the 1450s is immensely less likely than the time-frame pointed out by carbon dating.

Moreover, the manuscript is small and of average quality. It does not look like something made for a Prince-Bishop, but like a notebook made for personal use by an individual or maybe a family (something like a hausbuch).

The historical context, illustrations and other factors lead me to identify Ladin as the most likely language for such a work. 

The manuscript contains both Italian (e.g. swallow-tail merlons) and German (e.g. crossbow-sagittarius, color annotations) elements. Therefore I see areas between Italy and Germany as the most likely candidates (though other places are of course possible). The Ladin-speaking area is a possibility, but I don't see why it should be preferred to German-speaking or Italian-speaking border areas (e.g. the Canton of Lucerne, Northern Tyrol, Trento). I haven't seen any specific evidence suggesting the Ladin language and the corresponding area in particular.

But I can't find any probable reason to create a new script in any scenario, so whatever the truth it is likely to be improbable. 

Some additions to what Rene wrote:
In the early XV century many different scripts were created to be used in ciphers. The cipher manuscripts by You are not allowed to view links. Register or Login to view. are in some respects an interesting parallel for the Voynich manuscript, but You are not allowed to view links. Register or Login to view. were created to be used in diplomatic ciphers.

As discussed by You are not allowed to view links. Register or Login to view. (among others) another option is an artificial language: both Lingua Ignota by Hildegard of Bingen and Enochian by Dee and Kelley were supposed to be written with their unique invented scripts.
I personally like the phonetic-script idea, but it poses a number of problems for European languages: e.g. character entropy is too low and word-length distribution is different (in these respects, as discussed by Jorge Stolfi and others, some languages from East Asia appear to be closer).