I’m having the same problem too.
When attempting to upload any images, or change any plugin parameters I get the following error:
Bad Request
Your browser sent a request that this server could not understand.
Client sent malformed Host header
After examining my logs I found the following entry:
[Sun Dec 21 18:20:40 2008] [error] [client 84.12.106.26] ModSecurity: Access denied with code 400 (phase 2). Pattern match “\\\\%(?!$|\\\\W|[0-9a-fA-F]{2}|u[0-9a-fA-F]{4})” at ARGS:aktt_digest_title. [id “950107”] [msg “URL Encoding Abuse Attack Attempt”] [severity “WARNING”] [hostname “ianmckendrick.com”] [uri “/wp-admin/options-general.php”] [unique_id “mYKe0H8AAAEAACegG2IAAAAB”]
Does anyone know how to overcome this please?