I ran into this problem as a result of an incompletely installed extension. The extension,
chromium-lwn4chrome, was installed through aptitude. I've got no idea how some of its files ended up not being installed.
I finally found it by running
strace -ochromium.strace chromium
In the chromium.strace file, I searched for the string "anifest" and found the lines
lstat("/usr/share/chromium/extensions/lwn4chrome", 0x7ffd0c3bc520) = -1 ENOENT (No such file or directory)
access("manifest.json", F_OK) = -1 ENOENT (No such file or directory)
lwn4chrome, I found
open("/etc/chromium.d/lwn4chrome", O_RDONLY) = 3
Investigating that file, I found
export CHROMIUM_FLAGS="$CHROMIUM_FLAGS --load-extension=/usr/share/chromium/extensions/lwn4chrome"
but there was no file or directory named
/usr/share/chromium/extensions/lwn4chrome on my system. I purged the
chromium-lwn4chrome package, and the problem went away. I then reinstalled it (because I was curious) and this time everything worked.