atzakas

Problem fixed. It had to do with ModSecurity. Here’s an explanation I found on-line:

ModSecurity is an open source embeddable web application firewall, or intrusion detection and prevention engine for web applications. ModSecurity provides protection from a range of attacks against web applications and allows for HTTP traffic monitoring and real-time analysis with no changes to existing infrastructure, by operating as an Apache Web server module mod_security or standalone, and thus increase web application security. However, misconfigured or overly strict rule sets, ModSecurity may cause your website to return various errors such as HTTP 403 Forbidden error or access denied error, login problems, or HTTP 412 Precondition Failed error, or HTTP 406 Not Acceptable error and other false positive symptoms.

To make matter worse, the configuration of ModSecurity rules and filters have to be done manually. Although there are free predefined certified rule set which can be used with ModSecurity out of the box, however the rule sets may be not suitable for each and every environment and may interfere with the operation of websites or blogs, and customizing and modifying the rules may be too sophisticated or complicated for some users. And for some websites that hosted on shared hosting service, the mod_security may be enable by default without options. So in this case, the best solution or workaround for mod security related issues is to disable mod_security filtering and rules.