A WordPress installation consists of a WordPress Core, a theme, and usually one or more plugins. Plugins can often cause conflicts resulting in a wide variety of problems that can be hard to diagnose.
Check for conflicts using a plugin:
If the website is live and has many visitors, you can use the Health Check plugin as it allows you to run a conflict check without affecting the live website.
You can get the plugin from here.
Conflict check - no plugin required:
Deactivate all plugins.
Try to reproduce the issue from a New Private Window / Incognito mode.
Is there still a problem?
If yes, then it's not generated by a plugin conflict.
If no, then activate each plugin one by one, checking the website for the issue after each activation. If it reappears after activating one of the plugins, it means that plugin is the culprit.
A plugin conflict may look like this:
How to avoid plugin conflicts:
Never install a plugin to a live website - even if you used the plugin in the past without problems. Install it first on a staging website, you can check this guide for setting up a development environment.
Take time to test the plugin before upgrading it. After you set up the staging environment, upgrade the plugin there. Test you website and if everything works fine, do the upgrade on the live website as well.
Make sure the plugin will continue to be supported. Make sure there are enough reviews and recent updates, too. You will run across a lot of abandoned plugins on the repository. If you don’t know the developer will keep the plugin updated for a reasonable amount of time, it’s probably a good idea to steer clear. Additionally, it’s a good idea to go do a plugin audit every now and again to make sure that none what you have installed are out-of-date. If they are, it’s time to do some research to find a replacement before conflicts start to happen. And they will.
When you find a plugin conflict:
Check plugin's support forums and documentation:
Go to official and WP.org forums, docs, GitHub and WP repository pages for each plugin you have installed. You should be able to track down help for your problem (or find someone who already has):
The first step when a plugin is conflicting with WordPress / themes is to reach out the developer of the plugin and get informations about the conflict. The chances are you are not the only one who experiences it, so most likely in the plugin's support page you will find the status of that bug.
Another way of dealing with it is, debug the errors yourself. WordPress has an article about how to do that here, that will get you started.