Ajax error while uploading or removing file through managed file
MaxRequestLen sometimes gets lowered as a result of server/module updates. Also, adding modules that add to the request length can also introduce this problem. This is why disabling modules can sometimes "fix" the problem.

See http://drupal.org/node/1015938#comment-7412202 for a solution that involves updating your vhost file, eg:

&lt;IfModule mod_fcgid.c&gt;
  # 50MB
  MaxRequestLen 52430000
&lt;/IfModule&gt;


Or try switching from FastCGI to mod_php or php-fpm (if you're using FastCGI).