Sink and Bathtub Interchange: Water Flow Issues and Debris Appearance after 10 Years

Hello. Recently, water from the sink began to run out in the bathtub. Similarly, when I pour water in the bathtub, it breaks it into the sink. Sludge reminiscent of cement mortar and small pebbles comes out with the water; / Siphon cleaned in the sink, and in the bathtub too. What could be wrong if everything went well for about 10 years before?

No drain to the riser.

Maybe someone poured some leftovers after renovation and hoped that it would run out.

The outflow of the washbasin and bathtub meets somewhere, from here and beyond is the cause of the described phenomenon.

Okay, thanks gentlemen for the response and help. You will have to go down to the basement, see and dismantle there, because sticking the wire from above in the apartment probably will not come out. Something goes up, or I call a plumber, because I even have a problem with the seal folded. This is not a job for me, I get nervous too easily, vulgarisms fly. Stench, heavy access under the bathtub (one removable plate), one million fittings, seals, muck, flooded bathroom, full of towels for washing just wet paprika. Massacre some ...

But how do you pour water into the bathtub and knocks you out in the sink?
Uphill? Maybe this is not a washbasin but a Turkish crumb?