I have recently upgraded my clients Magento 1.9.0 Multi-Store from PHP 5.3 to 5.6.
After the upgrade, we have noted some strange behaviour across the front and backend of the website.
Works fine, but intermittently will throw "Invalid Form key, Please Refresh The Page".
This is very intermittent & I have followed all of the general advice posted on the Forum & StackOverflow without any success.
This issue can trigger at any point, saving tracking, product changes, CMS pages or general configuration settings.
Pressing the browser "Back Key" with reset the Form Key & allows me to carry on using the admin panel without logging in.
Once again, all of the issues are intermittent - so very difficult to test:
We have noted that on occasion customers add a product to the cart, but the cart displays "No items". When navigating back to the website the product is THEN added to the cart.
We have also noted that on the checkout, customers have occasionally been redirected back to the cart when "Submitting Order".
This does not empty the cart & on the second attempt, the order will proceed correctly.
All system, exception & server logs are clean. No records of any errors, notices or warnings.
This is a very tricky bug as it is very intermittent, which makes it almost impossible to test & debug, does anyone have any suggestions?
Yes I am using a Full Page Cache module:
Could this be causing the issue?
Before I start disabling modules, can you please explain why the cache could cause issues with Cookies & Sessions?
It doesn't seem related?
Mostly caching module rewrite session file so may be related to this.