WordPress Critical Error Caused by Plugin/Core Mismatch

This is a walkthrough of a real WordPress critical error I diagnosed and fixed on a site I manage. Site details have been left out; the technical sequence below is exactly what happened.

I got the “There has been a critical error on this website” message on a WordPress site that had been running fine the day before. Nothing had been touched manually — no theme edits, no manual plugin installs. That’s usually the first clue that an auto-update is behind it.

For the general causes and fixes for this error, see my full guide: WordPress Critical Error: Causes and How to Fix It. This post covers how that troubleshooting process played out on an actual site.

The Situation

Plugins on the site were set to auto-update, but WordPress core was not on the latest version. Overnight, one or more plugins updated themselves while core stayed behind. The next morning, both the front end and wp-admin were showing the critical error screen.

Symptoms

• “There has been a critical error on this website” on both frontend and wp-admin
• No recent manual changes
• Site was working normally the previous day

Step 1: Regaining Access

With wp-admin inaccessible, I connected via FTP, navigated to wp-content/, and renamed the plugins folder to disable every plugin at once. The site loaded again immediately, confirming a plugin was the cause rather than a core file or server issue. I renamed the folder back to plugins, which restored access to the dashboard with everything showing as deactivated.

Step 2: Isolating the Plugin

From the Plugins screen, I reactivated plugins one at a time, checking the site after each. Most reactivated cleanly. The error came back the moment I reactivated a redirect add-on for Contact Form 7. That confirmed the specific plugin responsible, so I deactivated it again and left every other plugin active.

Step 3: Checking for a Pending Core Update

Before doing anything further with the flagged plugin, I checked the Updates screen and found a WordPress core update was pending. Given the plugin had auto-updated recently and core hadn’t, this pointed to a version mismatch as the actual root cause — not a bug in the plugin itself, but incompatibility between its new version and the older core it was running on.

Step 4: Backup, Then Core Update, Then Plugin

Rather than immediately reactivating the flagged plugin and hoping for the best, I followed a specific order:

1. Took a full backup of files and database.
2. Updated WordPress core.
3. Confirmed the site was stable and loading normally with core current and the flagged plugin still off.
4. Reactivated the redirect plugin.
5. Updated the plugin to its latest version, since core was now current.

This order mattered. Reactivating and updating the plugin before core was current would likely have reproduced the exact same fatal error — the plugin’s new code still wouldn’t have had a compatible core to run against.

Step 5: Verifying the Actual Fix

With core and the plugin both updated, I didn’t stop at “the site loads.” Since the plugin in question handled form redirects, I submitted a test form and confirmed the redirect fired correctly and the connected notification email arrived. That’s the real confirmation that the fix worked, not just that the critical error message was gone.

What This Case Confirms

Two things stood out from this specific incident that are easy to miss when you’re just trying to get a site back online fast:

• A plugin auto-update conflict isn’t always about two plugins fighting each other — it can just as easily be a plugin outrunning an outdated core.
• The recovery sequence matters as much as identifying the cause. Fixing the root cause (core version) before reintroducing the flagged plugin avoids reproducing the same failure a second time.

How to Prevent This

• Keep WordPress core updates enabled or update regularly
• Avoid enabling auto-updates for all plugins on sites where core is not updated frequently
• Use a staging site for testing major updates
• Take regular backups before updates run

If you’re dealing with this error right now, the full breakdown of causes and fixes is in the main guide: WordPress Critical Error: Causes and How to Fix It.