Well, many reasons may casue this error, some maybe related to the theme while others not, anyway, here you can find the major reasons for this error:
Problem in the web server configuration
Whether your web server is running Apache, nginx or IIS, any incorrect configuration may casue this error, so in case you are running your website on a VPS that you can manage, we recommend having a look on your server configuration settings.
Most probably you are hosting your website on a shared hosting, or your VPS is managed by any hosting/server-management company, in 99.9% of cases the company will blame the software installed for such an error, so please go throw other recommenditions below beofre contacting them regrading this issue.
Incorrect file permissions
In general all your WordPress script, themes and plugins permissions need to be set at 755 for folders, and 644 for files. Setting permissions either too strict or too loose often cause this and other errors.
Corrupt files or files that contain errors in code
And this file maybe:
.htaccess file
One of your plugins files
In the theme
Related to the WordPress script files itself
PHP memory limits set too low