Invalid characters in Url?

Hello.

i am a beginner using LoadUI.

i have tried to create a simple load test, checking some ajax function.
i use the following url:

http://localhost:1912/Statistics/AddSte ... erialized=[{"Id":8,"tutorialID":13255,"stepId":102993,"stepIndex":1,"totalSteps":2,"playGuid":"1352019882465-13255-5b34567a-32b6-bd82-32c7-76274cbf2a3e","TicksSince1970":1352019887591,"StepStatus":2},{"Id":11,"tutorialID":13255,"stepId":102994,"stepIndex":2,"totalSteps":2,"playGuid":"1352019882465-13255-5b34567a-32b6-bd82-32c7-76274cbf2a3e","TicksSince1970":1352019890398,"StepStatus":2}]&domain=www.google.co.il&source=0&customFieldsSerialized=[]&browser=Firefox+16.0&os=Windows&_=1352019894217

yet nothing happens.

if i cut it just to:

http://localhost:1912/Statistics/AddStepLogs

it works (technically, not functionally).

suggestions?