Hello @MaWa, Welcome to the /e/ users forum.
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even lacking from /e/ “official” documentation that is not 100% fiable for years… despite multiple reports, removing all screen lock, SIM lock, carrier lock and Gogol account before switching to alternativeOS is a necessary pre-requite !
this is not relating to bootloader status, but to factory-reset.
you will have to reinstall FairphoneOS (relocking is not needed), then remove all screen lock, SIM lock, carrier lock and Gogol account, and finally install /e/OS
How to manually install FairphoneOS
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[HOWTO] a Clear Guide to install /e/OS on a Fairphone