404 when trying to add Conditions to product rules

[RossCC]

Magento CE 2.2.6

Trying to add a new catalog price rule, when you get to the "conditions" section, select a condition from the drop-down, Magento say "This won't take long" and then redirects you to a 404 page.

There is nothing related in the system.log or exception.log.

The URL on the 404 is 

https://www.mystore.co.uk/admin/catalog_rule/index/index/key/10282df4f760d9033b8fd26a4da65f3f89c5e3f...

This issue has only been occurring since we moved the site from a self-hosted dedicated server to a Sonassi Magento Stack.

Anyone got any bright ideas?

[Tarandeep Singh]

@RossCC 

You must look into server (Nginx/Apache) or PHP log as well to get a hint. You must also try to connect with hosting provider and they may tell you where the issue is. Sometime their internal firewall or server setting can also cause such issues.

Problem solved? Please give 'Kudos' and accept 'Answer as Solution'.

[RossCC]

Hi all

This turned out to be an issue with an incorrectly configured WAF / CDN setup.  All sorted.