LEDs flashing but board is not booting


I have a problem with an own, custom cape. When the cape is not connected, the BBB boots well. When my cape is connected all four user LEDs are flashing with a delay of about 1 sec but the board doesn’t boot.

My cape already does take care about SYS_RESETn signal, I do not drive any (HIGH) signal into BBBs pins as long as SYS_RESETn is not HIGH.

So…what could it mean when all these LEDs are flashing?