I could identify 2 issues:
- first: nand_init (where I placed a printf) is not just called in start_armboot(), but also in s_init() - and I guess this is called before initialising Serial port -> crash
- second: It makes a difference wether I flash the xloader from a running System (xloader+uboot) or after loading the same uboot with TI's Flash tool (if uboot isn't work anymore). I guess this is due to different ECC handling.
Nevertheless thanks Biser Gatchev for your input!