Second pic is of Passthrough mode, not 15khz. If it was true passthrough wouldn’t it be 15khz or is the projector picking up the signal but processing it as 60khz?
vertical horizontal refresh rate with frame rate. That 15kHz figure refers to the vertical horizontal refresh rate, or the number of times per second that the monitor writes the frame a line to its screen, and the 60Hz figure (Not 60kHz) refers to the number of frames fed to the monitor per second.
vertical horizontal refresh rate also hints at which video modes are supported, so when BuckoA51 suggested that your projector is 15kHz, he was suggesting it may be limited to SD modes (240p/288p/480i/576i), though I find that hard to believe when it has HDMI-input.
For troubleshooting, I’d try the HDMI->VGA converter with a different HDMI device, but the same cable and otherwise the same settings on your projector. If you don’t get the interference, then it might be a quirk with either the OSSC or how the converter treats the signal from the OSSC. If you <i>do</i> continue to get the interference, you should try a different VGA cable (preferably a good, beefy one).
You may also want to look into getting a secondary scaler/video processor that can normalize the output from the OSSC, and perhaps upscale it to 1080p or whatever resolution your projector supports, so you can simply feed it to the projector with HDMI.