As Jack says! You bought a capture card that does only capture broadcast resolutions and framerates. 1280x1024 is NOT amongst them.
1080P50/60 should work. But might be that you send the wrong SDI format. In 3G-sdi there are a few things that can prevent you from having video. Level A vs Level B and Payload ID issues if the wrong SDI chipset is used.
If you would have used a BMD Micro HDMI to SDI you are able to set the Level A vs B. and we are sure there is a official SDI signal on the output. Instead of China Garbage. And really that HDMI > SDI micro is not a lot more expensive.
Standards
SD Video Standards
525i59.94 NTSC, 625i50 PAL
HD Video Standards
720p50, 720p59.94, 720p60
1080p23.98, 1080p24, 1080p25, 1080p29.97, 1080p30, 1080p50, 1080p59.94, 1080p60
1080PsF23.98, 1080PsF24, 1080PsF25, 1080PsF29.97, 1080PsF30
1080i50, 1080i59.94, 1080i60