Just a few days ago I was struggling with my mirror giving me the above error. What was bugging me was that using another Manjaro mirror removed the error.
I found an old topic on Arch forum and a bugreport on Arch bugtracker mentioning the same error message and while it was closed as not a bug it gave me some idea on how to solve my issue.
The solution is only a workaround I still have to figure out why one mirror bugs out and another doesn’t.
I was kind’a hoping you had a third-party mirror in your pacman.conf but you don’t - at least not this pacman.conf.
On to my workaround … which I provide with no guarantee of success or solution - only as a suggestion.
Change the line in pacman.conf reading
SigLevel = Required DatabaseOptional
To
SigLevel = Required DatabaseNever
What the change does is telling pacman to never look for the signature file for the databases. As there only very rarely is a signature file for the database e.g. community.db there is no need to have pacman looking for it.
My issue came after I added my own third party repo on the same server as my official mirror - so I am speculating on the coincidence - and how small the chances are to get this issue - not on one system but two 
- and as you see later on - a third arm based system 