Clogged Drain?

Did an inspection 3 months ago. Drain at the kitchen sink was cracked at the elbow an was unable to fully test the drain at that time. Recommended repairs to drain crack. They just moved in in the past month, did some renos in the basement area and now they have a leak.
Haven’t seen the leak yet but the buyer told me that the leak was at the screw cap in the downspout under the kitchen sink. He also mentioned that there was some backpressure in the drainage line. I’ve heard of this if the vent line starts to fill with water because the drainage line is clogged. Could this be what is happening? All the other drains had good drainage flow. The house is only 9 years old.
Any ideas to help figure this out for him? Meeting with him in the morning to look into it further but would like some ideas when I meet with him.

Alen…probably just a normal plug at the first 90 after the trap…very common especially after a while not used…Draino or a small snake will most likely cure but really not your problem as you mentioned issues with the drain at Your inspection…jim

Thanks Jim,
That is what I was thinking that it was clogged but needed to hear fom others. I was second guessing myself I suppose.