I had a Belgian Dark strong I brewed last year that inexplicably failed to attenuate fully. No matter what I tried, it stalled. It is a beer I have brewed man, many times and I just could not explain. Searching for an explanation I began wondering if it didn't have something to do with the batch of Dark Candi syrup. Since this is a "waste product" from the candi industry in Belgium (or, at least that is what it says in BLAM) could it have been a batch of candi syrup that was dextrinous? I never could come up with an answer. the yeast health was over 90% viable going in and fermentation seemed strong then just stalled. I re-pitched entire conical cone fulls of both Belgian and American Ale yeasts and never could get it to budge another point.
If you believe CSI's marketing blurbs, they are making a syrup that is completely different than the big Belgian candy companies.
I've never really considered this though. I have brewed with syrup many times and while I've never had this issue, I don't consider it outside the realm of possibility. These days I'm looking to explore some different recipes for monastic ales using my favorite brewing methods...
I think Keith was referring to the other syrup company, Dark Candi. But I still have a hard time believing it was the syrup.
I'm wondering if it was the pressure in the fermenter that was causing the problem for the OP. We've found that the Dupont saison strain needs to be open fermented to obtain a decent FG. Maybe 3787 has some of the same characteristics?