Please help us help you and fill where relevant:
Your LimeSurvey version: Version 6.4.1+240108 
Own server or LimeSurvey hosting: Own Server
Survey theme/template:  Bootswatch Theme (TrueWiring_Main)
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Hi! We've had a survey completed and up and running for awhile, but suddenly when someone submits the survey it shows a PHP warning. We don't mess with PHP settings when setting up the survey so we are unsure of what is causing the issue. Can someone please review and help? This is impacting at least 2 of our surveys currently. I've attached a PDF of all the PHP errors listed.  

1. Remove debug www.limesurvey.org/manual/Debug_mode
2. Remove the the admin notification email : www.limesurvey.org/manual/Notifications_..._Notifications_Email
3. Fix your SMTP issue : www.limesurvey.org/manual/Global_settings/en#Email_settings (you can send a test email)

When 3 is fixed : you can add again the admin notification email