We are doing support work for a Backdrop CMS / CiviCRM site that we did not build. The site has been moved to a Nginx server and is generating a "Session invald. Please reload and try again" error when trying to process a contribution through CiviCRM. 

There is nothing useful in the logs.

Looking for tips on how to debug this problem. 


The first thing I would check is that the BASE_URL value in civicrm.settings.php matches with the actual URL. For example, if the new site redirects to the 'www' version of the URL but BASE_URL value is set to the non-'www' version, that could cause an error like this.

We checked the base URL in both the settings.php and the civicrm.settings.php file. In both cases it includes the www as we are using the www in the URL. 

We have made sure that everything is fully up to date. 

Any other ideas?