I am experiencing the same problem described here: Fail to boot: Codepage not found.
My error is:
FAT-fs (sdx1): codepage cp437 not found
My fstab mount command for the device is:
LABEL=ESP /boot vfat rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro 0 2
The above is automatically generated by a script and it hasn't changed recently. The problem started recently.
I have already run
mkinitcpio -p linux and it completes as expected without any errors. Other systems that are configured identically (afaics) do not have this issue.
I have checked the wiki as suggested at the comment by Gilles on the other question, but I don't find the specific problem.