Did you try remapping the jump key?
Can you jump at all on platforms? (ie when standing still, or moving but not close to an edge)
See the topic
jumping in a D2:ED forum, which has a couple links to other possibly relevant topics.
Are you playing the retail PC version of the game using a no-DVD hack? IIRC one of the copy protection mechanisms that could be triggered if no DVD was detected was disabling jumping (though I would assume that would be all jumping, rather than in certain circumstances).
