like the title says, ive tried to get 1.3+ forge to work, but on the old launcher, as expected it doesn't but it does on the newer one. is my circumstance a one off, or is it made like that. if so is there any chance that it will work on the old launcher?
this error has persisted for me too, all forge updates excluding upto version .16 do not show up in the launcher, all the versions after that run the same error that OP has stated