George,
yes, you were right, the struct has three uint64 members, so the compiler assumes it to be aligned to 8 bytes.
The problem in my case was that I have to copy the struct to a contiguous memory region in order to send it via ethernet. The function where I experienced the problem just takes a pointer to the next "free" position in this send-buffer. At the moment, we cannot guarantee any alignement of this buffer.
I resolved the problem by assembling the struct locally (on the stack) and copying (casting memcpy-parameters to (uint32*) ) it to the right place.
Thanks for your help,
Roman