I had posted this on another thread and thought it would be useful to do it again here.
I don't think the error is anything to worry about, at least not for now. It sounds like there may be some moisture and condensation that had gotten into the camera. There is a good chance, based on the lens that you're using as well as users' reports of the looseness of the lens attaching to the Mark III, that water may have seeped into the lens mount area. You can check this by carefully removing the lens by sitting the camera with the lcd screen faced down, twist it carefully and take the lens right off. If there's moisture around the metal ring, then it's a good indication. Moisture may have gotten into the contact area where the lens and camera communicate electronically, resulting in the error message. The camera and lens may simply need to be dry out thoroughly. Try the rice technique first, if it doesn't work then I would suggest you contact an authorized Canon dealership directly. I am no technician, but based on a lot of reading and experience, this may help remedy the issue.
This is a proven technique that has been confirmed by many photographers as well as other electronic repair technicians. I have this worked on my iPod which was accidentally dropped in water.
For best results, do this right away. The longer you wait, the chance of water getting into the electronic parts is much higher.
(1) Make sure the camera is turned off. Take the memory card and battery out of the camera and store them in a dry place. If you have images on the memory card, refrain from inserting them into a card reader right away. Instead let it sit in a dry area for a few hours before attempting to transfer the images. Don't bother to take the lens off. Doing this, you take the risk of getting water into the camera mount. Plus you'll want the lens to be included so it can dry together with the camera.
(2) Take a small dry bath towel, and in pressing motion, dab the camera until it's dry or as best as you can dry it. Don't wipe it as the wiping motion may push water into the crevices.
(3) Next you will need to get some white rice. Yes, you heard right. Rice. Long grain, Jasmine, whatever. Take a sturdy dry plastic bag, carefully put your camera in in (if you have more than one camera, use a bag for each camera). Pour enough rice to cover the entire camera. Now close up the plastic bag completely, either by hand tying the open ends or use a twist tie or rubber band. Let the bag with rice and camera sit in a dry place for a good 8 hours, or better yet overnight into the next day. Every once in a while rotate the camera in the bag, making sure most of the rice is covering the camera. You can do this without opening the bag. The rice will soak up all the remaining water and moisture.
(4) When you think the camera has been sitting long enough in the rice, untie the bag and carefully take the camera out. I say "carefully" because you don't want any rice powder to get inside, or under the lcd or crevices. Once the camera is out, use a brush, preferrably a lens or camera cleaning brush and brush off all the remaining rice still sticking to the camera. Use a rocket blower to blow the rest off the camera.
(5) Put the memory card and battery back into the camera. Turn it on. If no error blinking messages come up, you should be good to go. To be safe, now would be a good time to transfer the images onto your hard drive if you haven't done so.
Cross your fingers and wish for luck.