Unfortunately, while Format (NFormat is identical to Format for more than year now) maintains similarity to "printf" formating, there is a catch - in order to allow extending Format, the format is specified by ALL alpha characters after the % - so in this case, it is "%sscrgrp". You can delimit it using '`':
"%s`scrgrp.html"
BTW, it release mode, it would most likely worked as expected - if type is not found, Format starts trimming the identifier performing more searches until it founds %s (but of course, this is just to prevent unexpected surprises in legacy code).