Update fails with "keyring is not writable"

You have pacdiff files too.
Seems you do not follow the correct update procedures (checking the announcement thread).

Handle the pacdiff files and see if that solves anything.

I eeeh, actually searched the internet for a manjaro tutorial on pacdiff, I know there is one but have not saved it. The top link is MY question about it, read the thread and I think you will understand.

I recommend using meld instead of vimdiff to handle the pacdiffs: Support on how to use pacdiff

To me it looks like an update broke somewhere or that your system has not been updated in a loooong time.