I'm not entirely convinced that error is coming from XFMG, though, equally, I can't think where else it would be coming from unless you have any add-ons which modify any gallery functionality?
The only place in the XFMG code which uses the phrase: "please_enter_value_using_x_characters_or_fewer" is to do with Gallery custom fields but that shouldn't be relevant here.
I initially thought it could be related to the media titles which do default to 150, but the error message for that phrase is different.
So, all in all, I can't reproduce this. Do you have any add-ons that extend XFMG? Even if you don't, can you reproduce the issue if you disable all add-ons and try again?