Periodic (daily) Time Machine failures

When you access a share via AFP, the AFP service running on the NAS (called netatalk) creates a number of hidden directories in the root of the share beginning ".Apple". These directories are used to store Mac specific info. In reality, for the TM share that info is not actually needed - but it is created anyway as part of how the service works - and if it gets corrupted, or for example not upgraded properly when the NAS firmware is updated, then it can create issues accessing the share via AFP.

I have seen this happen before, but not specifically resulting in the error you report - but there are posts on other forums that show a similar error to yours and suggest a similar diagnosis. As the info will get recreated when you access the share again via AFP, and that there is no TM specific data stored there anyway, deleting those directories and allowing the AFP service to recreate them would seem a reasonable debugging step

The specific directories to remove (or just move) are:

/c/.timemachine/.AppleDB
/c/.timemachine/.AppleDouble
/c/.timemachine/.AppleDesktop

(It is possible you do not have all 3 - .AppleDB is the important one)

Good luck