The most likely cause is that the Rebel XS is still trying to confirm focus before releasing the shutter, even when the lens is set to manual focus. On Canon bodies, a blinking focus confirmation light usually means focus has not been achieved on the selected AF point.

So this may not be a lens failure at all: in MF, the camera can still expect you to manually focus until the selected focus point confirms. If you're off that point, or light/contrast is low, the shutter may not release.

What to try:

• Select a specific AF point.
• Manually focus on that exact point.
• Test in good light with a high-contrast subject.
• Try a different AF mode such as AI Servo/AI Focus to see whether shutter release behavior changes.

Because the lens worked normally on a T2i, the problem is more likely related to the XS body or its settings/behavior than the lens optics themselves. If the issue continues even when focus confirmation should be easy, the older body or its lens communication may need service.