TQM860L hangs after soft reboot

Hi all,

We've designed a custom board for an TQM860L module from TQ-Components.
Now the following behaviour occurs:

After a soft reset (from linux or bootloader u-boot) the board hangs. 
Only a hard reset (power cycle or RESIN# pin low of TQ-Modul) wakes it
up. 
In the evaluation board STK8xxL from TQ the modul boots normally after a
soft reset.

I assume it might be a forgotten pullup- or pull down in our custom
board.

Any hints which pins I must check?

By the way, how does linux handle a soft reset?

--
Dipl.-Ing. Jens-Peter Oswald