The following error message appears all over SharePoint on Site Contents after debugging has started and already ended.

General issue related by passing in the query parameter.
Tested in this case in Firefox.
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I think that's related to this: https://docs.microsoft.com/en-us/sharepoint/dev/spfx/debug-modern-pages#disabling-debug-scripts
Once you start a debug session, it is persisted through the browser session unless you reset it.
Thank you @andrewconnell - Confirmed - documented but awkward implementation.
The only question left. Why pages only in the _layouts/15 show this error message but none of the others. Considering this I think it is a bug rather than a wanted behaviour.
Especially look at the "errorLoadingDebugManifestTitle" as well as "debugManifestErrorDebugDismissButtonText" this seems more like an incomplete malicious implementation.
@johnguy0 ... can you address @StfBauer's follow up questions? This is related to the change that was made a while back that you were looking into...
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