If I use Davide's suggested code in WW2.19 I get the same error.
The fraction context was reworked for WW2.20 as part of the new Context extension system so that fractions would work well within other Contexts. I suspect that Davide's suggested code was written for the new version of contextFraction.pl. He'll have to chime in on whether there is an easy tweak to make it work for WW2.19.