My Raspberry Pi 4 has an issue when booting. I installed Retropie (not as dual-boot or boot) on it a few days ago. After i rebooted the issue started appearing. The screen goes from rainbow-phase to OS-choose-phase and then displays a few lines of starting services. Right after that it begins flashing quick images (of Retropie Emulator) and starts loop. I managed to capture a few on video with slow-motion. Long story short: all services start (Gitea, fail2ban, etc.), but the UI doesn't appear. SSH seems to be broken (config issue). Any ideas how i can fix this without wiping my whole SD-Card? Thank you in advance!
In general, when you do not have access to the Pi, the only solution is take the card out and mount it on another computer. On a Linux system, that is the easiest, and you may consider using a USB-booted Linux box for it (if you're on Windows).
First step after mounting the second partition of the SD card is to examine the logs. Normally, there should be error or warning messages in there. You should be able to see what the config issue for sshd actually is.
If you found the error, then you should be able to correct it, possibly by editing config files on the SD card. It is, at this stage, not really possible to tell you what is actually wrong.
You might consider just getting sshd up and running and do the rest of the repair via ssh. That could be a better option if the retropi installation did not provide any real clues in the logs.