Roberto Inzerillo wrote:
> I have been using Libreoffice 5.1 under Windows and had no problem with
> Math symbols before.
>
> But today, after installing and uninstalling a few programs, Libreoffice
> suddenly started displyaing some symbols in a wrong way.
>
> This is a screenshot of what I'm talking about:
> http://www.robertoinzerillo.com/temp/math_01.jpg
> The "vector" symbol over the letter "A" is being rendered as a series of
> squares 
> This is not the only symbols which has been corrupted.
>
> I already followed a few hints found in the internet, such like
> downloading and installing opens___.ttf file again; but had no luck 
>
> Any suggestions?
This definitely looks like LO doesn't know the OpenSymbol font or forgot how to use it.
Can you try the following?
In a Writer document, choose OpenSymbol as font, and then Insert > Special Character
You will get a table of characters. About halfway the table, just above a sequence of Greek characters (the second set of Greek characters) ΓΔΘΛΞΠΣ, there is a vector symbol. Can you insert that?
You can also check in a Formula with Tools > Symbols. (or Catalog, depending on the LO version). Choose symbol set Special, and click Edit. You should get the same table. See if it is there, and then Cancel.