There is serious bug in Blender plugin causing white edge on an image with transparency. Standalone render fine. In both cases "Premultiplied alpha" is disabled.
I attach both versions for comparison.
Edit:
I've found the reason of such behaviour. The Blender's Color management gamma was set to .04545 and Display device was set to sRGB.
Setting back Display device to None and gamma 1 brought back the correct alpha on renders.