Warnings and Notices in EE6

This actually started when I was upgrading a site from EE2. I got it to EE5 and started seeing some odd notices and warnings. So I upgraded to EE6 hoping to fix the issue, but the warnings persist, and I can’t pin down the cause.

Notice
Trying to get property 'site_name' of non-object

ee/ExpressionEngine/Model/Template/TemplateGroup.php, line 174

Notice
Trying to get property 'site_name' of non-object

ee/ExpressionEngine/Model/Template/TemplateGroup.php, line 174

Warning
Cannot modify header information - headers already sent by (output started at ee/legacy/core/Exceptions.php:120)

ee/ExpressionEngine/Boot/boot.common.php, line 477

I only see this when editing a template or initiating a system change, like adjusting a member group. Everything still works, but these warning messages persist. One odd thing is that I don’t see the error messages when editing a snippet or a variable.

I’ve run the server wizard, and it did not flag anything other than the lack of URL Segment Support.

I’ve tried adding a second Super Admin to see if it was an issue with the legacy member account, but that wasn’t it. I’ve tried disabling all add-ons and extensions, but that didn’t work.

Currently running EE 6.0.6 with PHP 7.4.19

Any thoughts?