Lexmark Optra T610 keeps saying "change tray one to plain paper"

Open the tray and make sure the green paper lever is set to letter mark, both at the bottom of the paper and the one on the side. And yes, if you haven't used it for 3 years, you should clean the dust that settled inside your printer over time.

You can change the language on your Lexmark Optra T610 by pressing right arrow Menu button 6 times -> select Printer Language (first option to appear), hit Select -> select preferred language, hit Select button to save.
Hope this helps. Thanks.

If your printer only prints black pages it will require the high voltage power supply to be replaced. this will fix your problem

Most probably the paper tray guide is not set properly. Here's a link to the Lexmark website to guide you on what to do when having an error like this. Just click on this link: http://bit.ly/c4AO17 . You can move the paper tray guide back and forth and setting it to the correct size.

sounds to me like a dead back up battery. I think you'll have to call a dealer to confirm this, and they would have to replace it for you

It sounds like they changed the Paper Type in the menu settings. I cannot remember 100% where it is on this model, but if you go through the menus, you should come across a Paper Menu option. In this menu, you need to change the Paper Type for Tray 1 to Plain Paper.

verify paper tray is set for correct paper. print the configuration menu and check the paper size for tray 1. If it does not have right size, examine the tray bottom to see if slider is in place

Your paper size settings for Tray 1 is probably wrong. Make sure it is set to Letter and the type is Plain Paper. If your Paper size is Universal, then either the paper guide in the tray is set wrong or the paper size sensor is broken. If the sensor is broken, you can disable auto paper size sensing so the printer will allow you to specify a paper size. To disable paper size sensing, you need to go into Config Mode. The other way is to send a PJL command to the printer to disable the function.

you can replace the paper in the main tray with the envelopes. this is the only way unless you have a secondary paper tray or manual paper feed that does not override the main paper feed

What happens is that the rubber roller that's supposed to pick paper up and load it in doesn't pull the paper from the tray anymore, hence it gives the paper jam error message. If you want you can replace that rubber roller to fix the problem, however first thing I do is to take that rubber roller out (remove tray and look underneath to find the roller) and turn it inside out then fit it back. This will give the rubber roller the grip that it missed and solve your problem.