After a lot of testing i accidentally found out that this error only occurs when the parameter "ro" is included in the grub boot-commandline. Without it, the system boots as desired.
Also, it might be worth noting that the overlayfs can be turned on/off by a boot-paramter (regardless of the system config's value): overlayroot="" turns it off and overlayroot="tmpfs" puts in into tmpfs. Regards, pd
