Hi @Bas88 welcome to the /e/ forum.
Here is another thread just like yours Fairphone 4 /e/OS installation locked in FastBoot Mode marked Solved found by following the hashtags fp4 + eosinstaller (you might also check out the hashtag Web Installer too.)
The answer is just follow the manual install instructions Install /e/OS on a Fairphone FP4 - “FP4”.
The phone is unlocked, the flash process has presumably run to a certain point … but failed to complete; there is no documented way to find a log of exactly the problem but the phone “safely” reverts to Fastboot as system and Recovery are non working.