Nope. There is no scientific/medical proof that grapes are a danger to gliders.
When I got my first glider I read the same contradicting threads and info. For a long time I refused to feed grapes not because I thought they were a risk but because there wasn't any proof either way.
Grapes ARE part of the bml diet and have been feed long term with that diet with no documented issues in the last 15-20 yrs (how ever long bml has been around)
Nowadays, I don't intentionally buy a bag of grapes, but there are grapes in the fruit cups I use for my gliders so I do use them for the gliders. Most of mine don't like them much anyway but I do have a few that are eating them when offered.