Thank you Andras, you pointed me in the right direction.
It was not non-ascii characters, it was the decimal character. I am french and we use the coma as a decimal marker rather than a dot. (An american would write 0.1 and a french 0,1)
When I was using QTVK the stlReader somehow was expecting the french format for decimals instead of the usual one (I tested replacing all dots by comas in the file and it can now be read). So I ll try to find a way for QTVK to not inherit regional settings.
It could be good maybe to change the stlReader in future versions to not be affected by this.