Seems like I’ve found the culprit: https://github.com/Magisk-Modules-Alt-Repo/trichromelibrary-squoosh

When I deactivate this module and try again, the error message disappears and shows up again when enabled.

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I don’t want bloatware and Google services on my phone. I want to install magisk modules and use root. If a phone doesn’t allow unlocking bootloader and doesn’t have decent custom rom (or GSI) support, there’s no way I’m buying it.

That’s what I’m using:

https://gitlab.com/mudlej_android/mj_pdf_reader

I would advise to use open-source software wherever you can. You (almost) never have paywalls, ads or trackers there ;)

You can use an OTP Generator like Aegis Authenticator from F-Droid. Afaik it even works offline, so there should be no risk if you strictly keep it offline.