I have experienced the same problem on my 2010 DK-40se/hst with KB-2485 backhoe. In my case there are 450+ hard hours on my tractor and the right side mid-mount loader bolts have loosened up and been tightened several times over the course of the hours to date. Recently after having my hired hand check all the bolts for tightness, as a routine part of our maintenance schedule and because of a noise that was clearly evident from that area of the loader we found all the bolts loose, on the right side and just at the mid-mount plate. Upon careful inspection it is evident the bolts are too short for the depth of the hole and they're having loosened at times over the hours has allowed the bolts to actually cut threads into the mid-mount plate as well as possibly cause some hole elongation. Additionally, by having tightened the factory supplied bolts numerous times in an attempt to stop the problem from occurring it appears that the lock washers have been cutting their way into the plate and creating a recess where the lock washer contacts the plate. And the head of some of the bolts has been mushroomed due to excess stresses caused by the plates ability to move against the bolts. The plate is clearly moving, because the paint is showing where the plate was painted at the factory and now the upper edge tilts toward the rear of the tractor and toward the ground enough to be easily visible. There is slight evidence of movement on the left side but hardly any by comparison.
So to make an attempt to remedy this serious issue we removed the loader completely, and left just the loader mounting posts and frame which extends back to the rear axle housing. We took out all 5 bolts at the right side mid-mount plate and replaced them with longer bolts, after using a dial caliper to determine the longest bolt that would fit the holes without bottoming out, and without wasting any of the hole's usable thread depth. This is a metric 8.8 bolt that is M15 x 1.50, ( I believe- I don't have the spec in front of me, but I'm fairly sure that was the size.) Note also: I do NOT have any shims in my setup like the OP of this thread mentioned and shows in his pics.
After discussion with my dealer, who says he has seen this type of problem with about every tractor manufacturer he has ever dealt with, and all brands he had sold over the last 50 years we determined, that for now the best way for me to solve my immediate issue is to install the new longer bolts with new lock washers and an additional flat washer between the mid-mount plate and the lock washer. I did just that, and before doing so used Brakleen spray and cleaned out each hole and air dried it before installing RED locktite to each bolt's threads.
The reason I chose to use semi-permanent Red locktite is because I'm hoping it will hold the bolts in place and prevent most if not all future movement. One potential problem is the fact that using a flat washer under the lock washer gives the lock washer less chance to grip something that is stationary, like the mid-mount plate. This could cause the bolt to loosen up when it might not if it were just locked to the plate as it was before. Hopefully the locktite will overcome this tendency, but I'll know soon enough if this is still going to be a headache area on the tractor.
I jacked up the entire loader post to where it was originally aligned from the factory install, with a floor jack, after loosening the front plate on the loader post to ensure even alignment. Then I tightened ALL the bolts to close to 75'/lbs.
I did not want to overtorque them because of the possibility of stripping out the threads in the holes in the frame.
My dealer said he just did a repair on a DK-45 where he had to remove even the loader posts and drill out oversized holes and insert studs and then washers and nuts on the studs. So far, this particular repair is holding. I'm trying to avoid going this root if at all possible for obvious reasons.
I suggest to ALL of us with these series tractors to make up a simple maintenance sheet where items like this can be listed, like grease intervals and checked off periodically like clockwork to keep these problems from becoming potential nightmares.
Hope this helps.
Let the bolt torque be with you!
