This is most likely camera shake, not a manual-focus problem. At 300mm, 1/125 sec is often too slow to hand-hold sharply, especially at a wide aperture where blur is more noticeable.

Try these fixes:

• Use a faster shutter speed.
• Raise ISO so you can keep that faster shutter speed.
• Use flash if appropriate.
• If the subject is static, use a tripod and a longer exposure.
• Use the self-timer or remote release so pressing the shutter doesn’t add vibration.

A photo can look sharp in the viewfinder or on the camera’s small screen but still show motion blur when viewed larger on a computer. So the issue is likely motion blur from camera movement rather than the camera being miscalibrated.