Restore /boot/efi
Solution 1:
UPDATE: since /boot/efi became corrupted I slapped together a vulgar script to recreate /boot/efi. Use at your own risk!
After finding the message above (by entering the manual recovery commandline offered and running dmesg | tail) I tried manual mounting. This failed as well.
Googling turned up that one should check whether the module nls_iso8859-1.ko could be loaded. So I tried running sudo modprobe nls_iso8859-1. This turned up an error about nort beeing able to open the dependency database. Recreating the dependency database with depmod fixed the issue.