I found the reason why it loads the wrong material strengths for some players
I started thinking that the reason might be because I am European and not American and thus use different number notations.
Especially the symbols used for digits is different, we use the , to signify digits while Americans use the . to do the same.
so a value of 1/2 would be 0.5 for the game but 0,5 for my computer.
I went into my computer settings and changed this arround to use the American settings and then I loaded a game with the glassworks mod.
And presto, normal material strengths and normal mining speeds.
But this does mean that proably all non-American symbol users will have this problem.
Alternatively if I replace all values in the materials.xml from 0.5 to 0,5 they also load fine and have normal strengths ingame.
So I now wonder if this also affects all other values using decimals in the various xml files in mods.