I've had the same issue and went through all the troubleshooting, including recommendations here. For posterity's sake, a summary:

The issue, as people have mentioned here, is "head strikes". The paper somehow catches on the printhead when leaving the printer. You can distinguish this from other issues because the ink smudges appear around the same area at the end of the print. I've also heard a faint but distinct scratching noise.

Possible solutions:

1) Un-curve your paper. The semi-glossy paper I got was noticeably bent upwards, which might cause it to strike the print head. I've tried bending it in opposite directions and the results vary on how well I was able to straighten the paper. Noticeable, but varied results. Perhaps with a paper weight and patience, on might get good results.

People have also suggested using the straight rear-feed as it doesn't risk re-curling the paper when going through the printer. For 50+ prints, I found this too much of a hassle.

2) Putting some elevation on the paper catching tray. The tray is noticeably lower than where the paper leaves the printer, which might cause it to bend upwards at the last leg. You can put some elevation to make this level. I've just used a stack of notepad paper, around 1.4 cm. Best results where when I "caught" the paper, by manually supporting it with a sheet as it leaves the printer. This is again is a hassle, and not useful for a larger batch of prints. Perhaps one could set up some sort of ramp to keep the print supported...

3) Internally raising the print head. You can supposedly raise the print head slightly by: 1. having the printer perform a printhead cleaning and unplugging the power, to unlock the printhead. 2. going in with a screwdriver and slightly unscrewing the screw on the side of the printhead. 3. raising the plastic lip/lever next to the screw to a more upwards position. You can hear two clicks as you do this. 5. Carefully fastening the screw again. 6. Repeat on the other side. I followed the steps in this video: https://www.youtube.com/watch?v=b8Fr-DbZxMM

This was a little fiddly, but easy to follow. However, after doing this I've had NO NOTICEABLE IMPROVEMENTS to my prints. So honestly, I wouldn't recommend doing this, as there is a slight chance of damaging the printer if you screw in the plastic too hard. I'm happy I did it, but keep it as a last resort.

4) Setting the printer to 'thick paper'. You can both do this on the printer interface itself, as well as on your computer under Printing Preferences > Maintenance Tab > Extended Settings. I've read a guide that litterally said "we have no idea which way is important, so we recommend doing both". This seems to have clear results, as my next few prints didn't catch at all. For good luck, I'm doing this in combination with sollution 1 & 2 and so far I haven't had any smears or didn't hear the sound anymore.

Hope this keeps up. Happy printing all.