For an excellent article on HMF by one of the world's leading authorities
on honey chemistry, see White, J (1994). The role of HMF and diastase
assays in Honey quality evauation.  Bee World. 75(3): 104-117.
 
In that article, he gives an overview of how HMF came to be used as a
"quality" determinant in honey.  HMF was originally looked for in honey as
a check against honey adulturation.  Invert sugar prepared with acid
contains variable amounts of HMF, depending on the conditions used.
 
As to whether HMF is toxic or not, White quotes the Registry of Toxic
Effects of Chemical Substances, published by the US Public Health
Service, which says that for rats, at least, the subcutaneous toxic dose
is 200mg/kg.  The proposed Codex limit for HMF in honey is 80kg/mg,
although there are cases of some tropical honeys straight from the comb
which have a higher HMF level than this.
 
According to White, the HMF level in a carbonated cola-type soft drink
(which is sweetened by invert sugar) is 456mg/kg, or about twice as
much in a 12 ounce can as the Codex limit for a kg of honey!
 
I presume the syrup you're talking about was produced by the enzyme
inversion process, which beekeepers in North America all know is
supposed to be non-toxic to bees.  If the syrup was produced by the
acid inversion process, on the other hand, beekeepers' experience here
in New Zealand would suggest that such syrup can be toxic to bees.