I'm with Leo. The only thing I've needed flux for is some very old non plated (i.e. bare) wire. And at that I had better luck cleaning than using flux.
My old 0.1" headers solder fine with the suggested solder.
However I've not seen the parts you referenced so I guess I can't comment on what you are /were trying to solder.
I'm quite sure if the OP tries the suggested solder they will have no issues with the parts shown in the photos.