It sounds like a hardware issue that would require a tech, but.....
Assuming it hasn't been corrupted by a Clone file from another device, or even the same device with a different firmware, it may work to simply run a verification test.
CWIS > Properties > Security > Software Verification test
The next step after that assuming it doesn't find/fix anything would be to wipe the machine with an Altboot no data backup, no clone, Which would require a tech (I am not allowed to post the instructions to do it here)
because then it gives the 09-586-00 error saying it's empty when it's not. Both errors have been occurring multiple times within days of replacing. And we're not using that much magenta.
Why are you replacing them before they empty, all Near end of life messages are just that, they serve as a warning to make sure you have one on-hand, not that you should be replacing them.
You can also disable the warning or just change it to a single day (Which is of course a guestimate based on usage)
CWIS > Properties > General Setup > Alert notifications > Low Supply warnings
Hi - We keep getting a 09-566-00 near end of life error for Magenta on the 7835. We've replaced the toner about 8 times in the past month, yet every time we replace there is plenty of toner left in the cartridge. All other toners are ok. We've also tried running the ATC developer calibration to recalibrate the magenta developer assuming it was not getting enough toner. Issue persists. Any other suggestions?