Really? The syrup? If it is contaminated syrup I suppose I could see my way to believing that.. Most people do not add pesticides or other toxic things to their syrup though..
Honey actually contains the same basic sugar units as table sugar. Both contain glucose and fructose. Granulated table sugar, or sucrose, has glucose and fructose hooked together, whereas in honey, fructose and glucose remain in individual units. Fructose is sweeter than glucose, which is one of the reasons fructose is used in so many food products today. However, fructose does not convert to energy as efficiently as glucose. Saying sugar is not good for the bees is like saying bread and butter is not good for people.. If that is all you eat for months on end, your exactly right, you will not be exceptionally healthy. The difference is that the bees WILL be bringing in their pollen and they will also be bringing in natural nectar, so the fact is, that feeding while they build comb is adding to what they get naturally, and the comb will get drawn out faster.. NO, I am not arguing that honey is better for bees than sugar, I never will, but supplemental feeding is also NOT going to cause problems like the OP is seeing.
Without being there, or having the bees tested, I am also going to guess they mayhave gotten into some pesticides. A neighbors garden? Depending on what it was and how much, it can disorient them, and you will find them crawling about as you said. I would keep a close eye on that and any other hives you have, and would, in fact supplement them even more with syrup and HBH if you do not have supers on in hopes of keeping them occupied while the pesticide disperses. Usually a week of feeding is enough, or if it rains to wash it off or away..
Spotting could also be Nosema, if you have Fumadil B adding that to the syrup you are feeding may be of aid. But Nosema is usually a bigger problem when the bees are confined during winter and early spring. What you are speaking of sounds very much like they have gotten into something they didnt want to/shouldn't have.
It's not just the sugar, it's the water as well. If using tap water to make the syrup, there could be chemicals in the water that'll harm bees, as well raising the ph. I have well water. Although I don't have to worry about chemicals, my water's ph is above 7.0. If I had to feed my bees more than a quart, I'll buy bottle water.