Lukas You need to grant body sensors permission to Google Play Services.
That fixed the sign-in issue.
Lukas I tried Google Fit and even that doesn't couny any steps for me.
I can reproduce this on GrapheneOS with Sandboxed Google Play. I had the phone on me for a few hours and likely walked a few hundred steps, while none showed up in Google Fit. However, as soon as I granted Play Services the Physical Activity permission in addition to the Body Sensors permission (I had already granted Play Services the Physical Activity permission while testing prior to this), both walk15 and Google Fit started counting my steps. They didn't start counting from zero, but actually started from a counter of several hundred steps. Perhaps the apps did actually record my steps, but they didn't show them in the app unless I granted those two permissions to Play Services? That seems a bit odd to me, but is my closest guess. In any case, these two apps seem to rely on Play Services permissions in order to function correctly.
Tested on
Pixel 8
GrapheneOS 2024090400
Play Services 24.36.60
Google Fit version 2024.08.02.01.arm64-v8a.release versionCode 2026857249
walk15 version 6.7.0
Interestingly, before I granted those two permissions to Play Services, I tested this app, which counted my steps without issues (needed to grant the app the Physical Activity permission, naturally).