Found that the new #RaspberryOS based on Debian12 sets the memory page size to 16kB (from the default 4kB) which breaks a bunch of apps including jemalloc (used in Fulcrum - how I found out), btrfs, all android apps, etc.: 
Check the value used with:
getconf PAGE_SIZE
Be aware if you are using a new image (default for the Raspberry Pi 5) the solution is to place the line:
kernel=kernel8.img
to the beginning of /boot/config.txt and restart the RPi.
Let us know if you have any thoughts on why we shouldn't use the kernel8.img on the #Raspiblitz by default: 
GitHub
Pi5 16K PageSize Kernel Incompatible Software · Issue #107 · raspberrypi/bookworm-feedback
This issue will be a public tracker for 16K pagesize incompatible software. I made a similar post in the bookworm beta forum thread but that is sti...
GitHub
Switch to the kernel8.img by default when the page size is 16k bytes · Issue #4346 · raspiblitz/raspiblitz
Jan 01 18:21:03 raspberrypi systemd[1]: Started fulcrum.service - Fulcrum. Jan 01 18:21:03 raspberrypi Fulcrum[1301304]: <jemalloc>: Unsupported sy...