Hello Tom,
I've seen this a few times, but would really like to figure out why it happens. The solution has been to uninstall and re-install Chrome. Once you do this, you should be able to add extensions again.
NOTE: When you do this, make sure to check the box that says: "Also delete your browsing data"
2.) Reboot your computer
If you have any insight as to why this happened, I'd love to hear it.
Thanks,
Teddy