OpenLiteSpeed WordPress cache mysteriously not working

The OpenLiteSpeed web server (OLS) and LiteSpeed Cache for WordPress (LSCWP) plugin provide a great way of both speeding up WordPress and handing large numbers of visitors.

OLS is an open source derivative of the LiteSpeed Web Server (LSWS), which delivers most many of the key features, including the high performance LiteSpeed Cache (LSCache). Whilst it can’t read  Apache configuration files like its bigger brother LSWS (and thus can’t be used with a hosting control panel like cPanel or Plesk), it’s great for working with a handful of sites configured manually

Whilst the LSCWP plugin has a lot of useful features which can be used even without the LSCache from OLS/LSWS, the main selling point is the integration with LSCache to deliver blazing fast page load times along with massive scalability under load.

I recently ran into a bizarre issue where the cache just completely stopped working for no obvious reason, which led to hours of pulling apart WordPress and OLS to try and work out why.

Aside from pages loading very slowly, the main clue was that the X-Litespeed-Cache header was completely missing, although the X-Litespeed-Cache-Control header was present as normal. This would normally mean some kind of issue with the cache storage location (/usr/local/lsws/cachedata/ by default, unless override by the storagePath configuration option in the cache module settings).
I couldn’t see any issues with the cache storage location, but tried adjusting it elsewhere anyway without any luck.

I verified that all of the settings for the cache module were configured as per those listed on the OLS wiki and eventually out of frustration deleted the whole cache module definition from the server configuration and added it back, at which point the cache started working again!

I’ve absolutely no idea why removing and re-adding the exact same configuration should make any difference whatsoever, but I have now verified identical behaviour on two different servers with completely independent configurations.