Ok, definitely could be a litterbox connection that one of the cats is fed up with - but ideally if you find the culprit you should have a urine sample checked for an infection or crystals.
But - it won't hurt to try to make things as appealing as possible at home. Even though you haven't had any problems with the setup until recently, some dynamic has probably changed between the cats -or- they have lost their patience with the litterbox not being clean enough. Many cats will tolerate dirty boxes for a long time, but tend to get fussy as they get older. They have done many studies to see what most cats prefer in regards to the litterboxes, this is what we recommend:
1. #1 most important issue to eliminate is box cleanliness. Absolute minimum to try to prevent (or address) problems is cleaning AT LEAST ONCE DAILY. Sounds like a lot, but with 5 cats I bet those boxes are pretty full on the days that you get around to cleaning them. If you can smell any bad odor when you go up to them, it is at least 10 times stronger to their little noses. They dislike it enough when they have to smell their own 'stuff', but it can really annoy them if they have to go where it smells like another cat. Doing this, far and away, usually makes the biggest difference on getting cats back on track.
2. The ideal litterbox 'math' is one box per cat, plus one extra. In a five cat household, that means finding space for 6 boxes.
3. Do the boxes have hoods? If they do, try removing them. Hoods can lead to an outhouse effect for the cat, concentrating odors. Also can make them nervous because there is only one way out.
4. Do you use box liners? If so, try stopping as these often absorb and hold odors in the box.
5. Have you changed litter brands recently? Normally we recommend to stick with what they've been accustomed to, as changing brands can upset them if they don't like the new texture or scent. Most cats, given a choice, usually prefer clumping/scoopable litter as it has a finer texture than regular clay. We also recommend using a fragrance free/unscented version, as some cats can be turned off by the scents of deodorizers. Remember the principle again that if you can smell it, it is 10 times stronger to your cat and can be overpowering.
6. Like Sicy said, you do need to carfeully clean spots where you know they have gone, with a carpet cleaner that has enzymes to break down those little particles that cats can smell when we can't. If you can stand it, while you are making the other changes with the litterboxes, try to keep the area rugs up since they are attracted to them at the moment.
Still is ideal to rule out something medical that is making them uncomfortable, but it is hard in a multi-cat household to know who is doing it unless you catch them in the act. Defintiely if you ctach the culprit, have a urine sample checked just in case. But it sounds like the box issues could be a big part of it. I don't want you to feel bad, most people don't know this stuff. And it is hard to know why they can go so long with no complaints, but often they will hit that breaking point and say, "I don't want to go in that stinky bathroom." That is my hunch anyway - worth trying to eliminate these factors...