@chopstick After trying a bunch of stuff in Discord it came down to the final option: factory reset. And for me, that actually did the trick.
Just in case you have similar issues: consider resetting first before wasting a couple days of trial and error. 
Hey guys,
let's start out with a big thanks for the great work! 
Board model: RPi 4 4GB
Recalbox version (build) : 7.2.2
Hardware (Micro SD, Power Supply, etc.): official Raspi PSU
Controller(s): different wired budget controllers
If your problem is game related
Which Game: all DC games
Emulator used: I think flycast is the only emulator available?
Is the problem reproducible (how): for it is always reproducable
I have already posted my issue on Discord. A friendly person told me that there seems to be an issue concerning Dreamcast in v7.2.2. I am here to make sure and ask if there is an official thread somewhere or even a ticket that I can hang on to?
By the way, I have not used Recalbox before v7.2.1. I couldn't tell if Dreamcast used to be ok and is now broken.
This is my original post:
After hours of trial and error including digging through various forum threads I still can't get Dreamcast to work at all. Hardware: RPi 4. I have tried various games including different formats. I have tried renaming the files. I have also tried different bios versions. The ones I am currently using are MD5-correct. I put them in share/bios as well as share/bios/dc because that's what the bios checker is telling me.
What happens for every game is return to folder view after a quick black screen.
Any tips where to look further? I am about to give up.
Thanks in advance