I suspect something with GZIP on server config or htaccess
that cause content_decoding_failed error,

its realy strange i diasble gzip, i only have this problem in backend no problem in front end

ahh htacess is the fault
`## These directives are only enabled if the Apache mod_headers module is enabled.

This section will check if a .gz file exists and if so will stream it

directly or fallback to gzip any asset on the fly

# Serve gzip compressed JS files if they exist
# and the client accepts gzip.
RewriteCond "%{HTTP:Accept-encoding}" "gzip"
RewriteCond "%{REQUEST_FILENAME}\.gz" -s
RewriteRule "^(.*)\.js" "$1\.js\.gz" [QSA]

# Serve correct content types, and prevent mod_deflate double gzip.
RewriteRule "\.css\.gz$" "-" [T=text/css,E=no-gzip:1]
RewriteRule "\.js\.gz$" "-" [T=text/javascript,E=no-gzip:1]

<FilesMatch "(\.js\.gz|\.css\.gz)$">
	# Serve correct encoding type.
	Header append Content-Encoding gzip

	# Force proxies to cache gzipped &
	# non-gzipped css/js files separately.
	#Header append Vary Accept-Encoding
</FilesMatch>

`

@micker If your issue is solved then please close the issue here. Thanks in advance.

My Joomla site had the same issue which was due to incorrect addressing of the log paths. The generated paths by Joomla's configuration.php file for some reason were not the proper ones for my hosting firm. By contacting them and putting in the correct path resolved this issue.

The symptoms were the JS and the CSS were not loading on the admin side of the site. It was not due to a htaccess/Gzip issue.