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Cortex-M33/STM32U5 - unprivileged mode in secure not working?!

I try run an unprivileged application in secure mode and get a strange behavior:

STM32U5 has TZ enabled, all application runs in secure mode.

1) LR = 0xffff_fff9, CONTROL = 1 => BX LR increments the SP, but all registers become 0 (including PC and XPSR!)

2) When setting CONTROL = 1 and executing SVC, SP is decremented correctly, but nothing is stored on the stack?

I check all errata I could find, but no hint.

If TZ is disabled, the system behaves normal.