Apache: "Additionally, a 404 error was encountered while trying to use an ErrorDocument" when file exists

Apache: "Additionally, a 404 error was encountered while trying to use an
ErrorDocument" when file exists

Here's my .htaccess in /var/www/avatars (I can confirm this .htaccess is
being read and executed);
ErrorDocument 404 /var/www/avatars/default.jpg
Here is what I get with a 404 page:
Additionally, a 404 Not Found error was encountered while trying to use an
ErrorDocument to handle the request.
Here is default.jpg:
https://coinchat.org:8001/avatars/default.jpg
Why is this error happening? It was working a day ago