"LT problem" - Mamiya 7 II

I used my Mamiya 7 II a couple of days ago outdoors in sunny conditions and the `LT` keeps blinking in the viewfinder despite bright lighting conditions.

The battery is fresh and the red warning light isn`t blinking, so the
battery power seems to be OK. This was with `AEL` selected. I try all other
modes (bulb, aperture priority, manual) but nothing has changed.

I tried to change the lens - but the same happened with another one. I
mounted my lens (80/4) to another body - and it works great, LT doesn't
blink. So the problem is in the body.
When the lens is out of body LT does not blink, everything is ok. What the
reason of this problem? And how to fix it?