Arm cortex-M4 crashes with hardfault

We get an exception and the lr is 0xFFFFFFF1. This means that we must cmp msp and psp. The address is now 0x2000E8D0. This is the stackpointer.

Than I get :

printf(" - Forced Hard fault.\n")
printf(" - MMAR holds an invalid address.\n");
printf(" - BFAR holds an invalid address.\n");
printf(" - Attempted to execute an instruction that makes illegal use of the EPSR.\n");
printf(" - The processor has attempted to execute an undefined instruction.\n");

What can I do now to catch the error further? I interpreted some values around the stack pointer but couldnt find any hint to an error.

The program chrashes after a function which is called from an IRQ.