I noticed a few people reporting this error, so I thought I would share what I had found.
My apache2 configurations (in /etc/apache2) were correct.
However, I found in
/var/www/html/admin/modules/ucp/htdocs
an
.htaccess
file.
The .htaccess file denies access to most files, but then selectively enables access on a per-file-type basis.
<FilesMatch "(^$|index\.html|index\.php|ajax\.php|\.(gif|GIF|jpg|jpeg|png|css|js|swf|txt|ico|ttf|svg|eot|woff|woff2|wav|mp3|aac|ogg|webm|less)$)">
This list of file extensions does not cover all the filetypes now included in the /assets folder - notably .otf and .map were missing. Adjusting that line to read
<FilesMatch "(^$|index\.html|index\.php|ajax\.php|\.(map|otf|gif|GIF|jpg|jpeg|png|css|js|swf|txt|ico|ttf|svg|eot|woff|woff2|wav|mp3|aac|ogg|webm|less)$)">
fixed my UCP access problems.
Reminder - .htaccess is a hidden file, so you need to do a
ls -la
to view it.