Unfortunately, viewports cannot be rotated in that way. Only the contents of the viewport can be rotated. The viewport frame (clipping) is always orthogonal.

Hmmm - I'm confused. It works in Model Space but the same Option (Scale, rotate) doesn't works in Layout Block?

Example with a 0.5 scale and 90 degree rotation

A viewport in QCAD allows for its contents to be displayed at a rotation angle. The contents of a viewport is the model space or part of it.

If you change the rotation angle in model space (while defining the area to be shown), you can see that the frame rotates, so that you can see exactly which part of the drawing will be displayed in the viewport and where it will be clipped.

If you change the rotation angle in paper space, the angle still applies to the model space shown. However, the viewport frame has already been defined at that stage and since it stays always orthogonal, you can now see the contents of the viewport being rotated.

Arguably, it usually makes more sense to adjust the angle in model space, while defining the part to be shown. But there are situations in which adjusting the angle in paper space can be useful as well.

Ok, thanks for this additional explanation. Understood.

"If you change the rotation angle in model space (while defining the area to be shown), you can see that the frame rotates ..."

This is what caused the confusion! You CAN'T see the rotation when a 90 degree rotation is used in Model Space. 0, 90, 180 and 270 degree behave visually on the same way. Every other rotation angle is clear noticeable.

Thanks!