When something does not work, the first question is what happened. The plugin writes its own log of opt-in attempts, autoresponder replies and errors, so you can see the answer without any server setup. You do not need WP_DEBUG for it.
Where to find it:

Free List Building, then the Settings tab. Scroll to the bottom, below Advanced, to the Debug Log section.
What the log shows:
- The newest 100 entries, newest first, in a dark text box.
- Each line starts with the date and time and a level, for example [INFO] for a successful subscribe or [ERROR] for a problem.
- Lines such as "Mailchimp: Successfully subscribed" followed by the masked address, or a provider's own error message.
- The number of entries on the right, for example 4 entries.
- When there is nothing to show: "No plugin errors found. Everything is working fine!"
Privacy built in:
- Email addresses are masked: only the first letter and the domain stay, with stars in between.
- API keys, tokens, passwords and license keys are replaced with [REDACTED].
- The error text from your provider stays readable, so the log is still useful for support.
Where the file is kept:
The log is saved in its own private folder inside your uploads folder, named listbuilderpro-logs- followed by a random code. The folder is protected so it cannot be opened in a browser. If WP_DEBUG_LOG is turned on in your site, a copy of each line also goes to the WordPress debug.log.
The buttons:
- Copy to Clipboard: copies the whole log. The button reads Copied! for a moment.
- Clear Log: empties the log after you confirm.
Step by step: send the log to support:
- Reproduce the problem, for example opt in on the funnel that is not working.
- Open Settings and scroll to Debug Log.
- Click Copy to Clipboard.
- Paste the log into your support ticket with a short description.
A real example:
A new subscriber does not appear in Sam's ActiveCampaign list. Sam opts in on the checklist funnel with a test address, opens the Debug Log and sees "ActiveCampaign: Successfully subscribed" with the masked test address and "to list 3". The plugin did its part, so Sam checks ActiveCampaign's automations next and finds the contact waiting there.
Tips:
- Clear the log before a test, then read only the new lines.
- Provider error text is shown as the provider wrote it, which usually names the fix, such as a wrong list ID.
- For a full picture of your server, see The Diagnostics Tab. For help, see How to Get Support.