I got that error too, but I didn't know what caused the error.
Anyway I managed to get my pi back up but it was quite a lot of work.
Before you start reading or doing the next steps: The way I fixed it is not the best way. I'm pretty sure that everyone who knows a bit more about linux would like to punch me for this but it worked for me and thats the only think that counts for me.
I can't promise that my way work for you. It's possible that I only had a lot of luck!
First of all I made a backup from my current sd card. A second sd card would be nice but is not necessary. But I highly recommend to perform a backup before continuing.
Next go on with downloading and writing the latest arch image from the download section. Boot your pi and perform a full system upgrade. Afterwards shut down your pi and mount the sd card on your computer. I choose a vm but that doensn't matter at all.
Navigate to /var/cache/pacman/pkg/ and copy the latest version of linux-headers-raspberrypi, linux-raspberrypi, raspberrypi-firmware to a safe place on your hard drive.
Now delete everything but /boot/, /lib/modules/ and /usr/src/ from the sd card.
Afterwards merge your previously made backup with the new sd card. Remember to not copy /boot/, /lib/modules/ and /usr/src/.
Now copy your copies of linux-headers-raspberrypi, linux-raspberrypi, raspberrypi-firmware to /root/, umount the sd card and boot your pi. Modules should fail to load as the module dependencies are from your old kernel.
Login as root and install the three packages in /root/ (pacman -U is the right switch). Install should fail as the files already exists in the file system. Install the packages either with the -f switch for pacman or delete the files pacman prints out.
After a reboot everything should be working again now.