Since no OTA update to version e-2.1-r-20240606407290-dev-ginkgo has been offered on the system so far, I have successfully carried out a manual update via PC and adb sideloadand, - also ended up in fastboot mode after the reboot.
After I installed recovery-e-2.1-r-20240606407290-dev-ginkgo and booted into e-Recovery, I am shown:
The system has been destroyed
Press powser button to shutdown
Next I installed PitchBlack Recovery Project and with it /e/OS-R v2.1, but again the system only booted into fastboot mode.
Now that /e/OS-R will no longer be officially supported in the foreseeable future (as of today, the latest Andrid security update is February 5, 2024), I successfully switched to crdroid 10.5 (Android 14) with MinMicroG.