Is there an abnormal condition that could have caused this fatal accident involving a stall?
Your question:


  Suppose that we take the pilot at their word, however. Is there any type of abnormal condition that might have caused a "feeling in the controls that something is not right", and which could cause the nose to drop with "none of the signs of a stall" occurring beforehand?


A couple of thoughts:


In a 60 degree level bank turn, stall speed increases by about 40%. 
See below for information regarding indications of an approaching stall in the G103. (from the G 103 C Twin III Acro manual)





The flight manual, section 4, indicates that with aft cg there is a stall warning 2-3 kts before reaching stall speed - (read) "There is a tail vibration which increases with continuing pull on the control stick." I think the note above (highlighted in yellow) means that with forward CoG, at max weight, there is no stall warning at all, i.e., the glider just stalls (nose drops without warning). I could be wrong, but this seems to me to be what the note means.

Here is a copy of the manual for the Grob G103 Twin III Acro: Grob G103 flight Manual

Here is a copy of the NTSB report I believe covers this accident: Grob 103 Accident - NTSB Identification: CEN09LA353