We recently had a very serious issue with a Joomla and CiviCRM site that the contributions pages (no matter what payment processor) were giving this error:

This page isn’t working somesite.com redirected you too many times.
Try clearing your cookies.
ERR_TOO_MANY_REDIRECTS

and we couldn’t figure out why. We also ran into this error from PayPal:

Error Payment approval failed with message :Security error Security header is not valid

and this from Authorize.net:

Payment Processor Error message :Security error Security header is not valid

Together with our client, we eventually figured out that we needed all these settings in Joomla global config:

I believe all I need to do is change the following in Joomla – global config.

1. Set Cache time to 30 or 60
2. Add the /donate URL to the following plugin
   · Plugins: System – Page Cache https://somesite.com/administrator/index.php?option=com_plugins&view=plugin&layout=edit&extension_id=424
3. Set Shared Sessions to YES (was no)

That fixed the problems.