Most likely an incorrectly oriented lens hood. Lens hoods for zoom lenses are leaf -like, with cutouts which are designed to avoid shading the corners. The most important bits of the petal should be top and bottom, if the hood is rotated it might be the corners of the image are being blocked by the hood.
So an intermittent shutter isn't going to be easily diagnosed. If the camera was particularly cold the mechanism that moves the blades out of the way may be slower. However what you're showing is a significant loss of exposure.
It honestly looks as if it's part of a lens hood or something stuck inside of the camera. I understand you said there was no lens ...
Based on the clarifications from OP for me this look like (random) problem with shutter. Will be good if OP can check in EXIF if this happen when taking fotos with mechanical shutter.
And it is wise to contact authorized service center where they can diagnose better the problem