What I tried,

On the test page -
Type : POST
Url : /openmrs/ws/rest/v1/hl7
Body Content : (the message you gave)
MSH|^~\\&|FORMENTRY|AMRS.ELD|HL7LISTENER|AMRS.ELD|20110805104142||ORU^R01|REl7wt78q9Pzlqe9ecJB|P|2.5|1||||||||3^AMRS.ELD.FORMID\rPID|||3^^^^||Patient^John^D||\rPV1||O|1||||1|||||||||||||||||||||||||||||||||||||20110805|||||||V\rORC|RE||||||||20110805104141|1^Super
User\rOBR|1|||1238^MEDICAL RECORD
OBSERVATIONS^99DCT\rOBX|1|NM|5089^WEIGHT
(KG)^99DCT||67.2|||||||||20110805

This results in :

error!
msg: error
errorThrown: Internal Server Error
req: [object Object]
Object
KEY	VALUE
readyState	4
success	[POINTS BACK TO done]
error	[POINTS BACK TO fail]
responseText	"{\"error\":{\"message\":\"Could not rea..."
status	500
statusText	"Internal Server Error"