If you are running Tinycore, you don't need flwm in OnBoot. So that is not the problem, and you can remove flwm from onboot.lst.
Boot with the options "base norestore" in the bootloader and see if it works.
If it works with "base norestore", maybe one of the extensions is causing the problem. See if you can figure out which one it is, and report it here so it can be fixed. You can experiment by removing extensions from /tce/optional, until you figure out which one it is.