macOS Sonoma: "Installer Progress" in menu bar during boot / red rectangles when switching tabs in Safari

My company is finally doing a macOS Sonoma pilot, so I upgraded from macOS Ventura to macOS Sonoma (14.1.2). Since upgrading I see the text “Installer Progress” in the menu bar during every boot. Additionally I see red rectangles when switching tabs in Safari. Both issues are mentioned in a Reddit topic, but I can’t find a solution there.

Does/did anybody else run into this on Sonoma? How did you fix it?

(I hope macOS Sonoma 14.2 will fix this, but I’m not going to install the Release Candidate to test that)