Overview
Recently I got this error message when I updated Redis Object Cache wordpress plugin on my site. As soon as I enabled Redis Object Cache, I got the error message: “There has been a critical error on your website. Please check your site admin email inbox for instructions.”
“A foreign object cache drop-in was found. To use Redis for object caching, please enable the drop-in.”

You can try to deactivate redis object cache plugin or delete it completely and then re-install it to check if it fixes your issue. Try to Flush Redis Object Cache as well if the wordpress redis object cache plugin allows. If you are using Ubuntu server where you have installed Redis then you can use also use below steps to fix this issue:
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Solution
To be able to fix this issue, you can first remove the plugin completely from WordPress Site by deactivating it and delete it. Then login on your Ubuntu server and run below command to remove redis server:
Remove Redis-Server
sudo apt-get purge --auto-remove redis-server

Check Redis-Server Status
Once the redis-server has been removed, you can check the status of the it by running below commands:
sudo systemctl status redis

You will not be able to get any response as the redis has been removed from your Ubuntu server.
sudo redis-cli ping
After running redis-cli ping command you will not get any response as the redis server has been removed from your Ubuntu Server. If its installed and working you can expect PONG message after running redis-cli ping command.
Update the server
sudo apt update
sudo apt upgrade
Install Redis-Server
sudo apt install redis-server
Reboot server
sudo reboot
Check Redis-Server Status
sudo systemctl status redis

Reinstall WordPress Redis Object cache plugin
Now, you can re-install Redis Object cache plugin and click on Activate to Activate the plugin:

Now, Enable Object Cache again and this time the status message should show as Connected, Drop-in as Valid.

Conclusion
Redis Object cache can definitely speed up the performance of database queries to reduce the load on mysql or mariadb database. It also reduces the response time of the site. However, sometimes you may encounter issues when upgrading Redis Object Cache plugin and therefore the simplest way to fix this is deactivate and delete the wordpress plugin, Remove and Re-install Redis server from Ubuntu. Server Restart is advisable but not mandatory. For Downloading Redis Object cache, you can also click on below link: https://en-gb.wordpress.org/plugins/redis-cache/.