Debian hangs on reboot

[hkramski]

Most of the time when I reboot my Debian system (current SD card from the shop), the reboot process hangs here:

[    0.624164] [uart]: used uart info.: 0xc1
[    0.628910] [uart]: serial probe 0 irq 33 mapbase 0x01c28000
[    0.655132] sunxi-uart.0: ttyS0 at MMIO 0x1c28000 (irq = 33) is a U6_16550A
<6>console [ttyS0] enabled
[    1.386892] console [ttyS0] enabled
[...]
[    1.496824] G2D: head:c9000000,tail:ca000000
<6>G2D: Module initialized.major:250
[    1.504423] G2D: Module initialized.major:250
<6>brd: module loaded
[    1.511763] brd: module loaded
<6>loop: module loaded
[    1.520414] loop: module loaded
<6>sw_

and I have to power cycle the board.

A cold boot at this point always gets successfully beyond this point with:

[    1.520433] loop: module loaded
<6>sw_ahci sw_ahci.0: controller can't do PMP, turning off CAP_PMP
[    1.529964] sw_ahci sw_ahci.0: controller can't do PMP, turning off CAP_PMP
<4>sw_ahci sw_ahci.0: forcing PORTS_IMPL to 0x1
[    1.541218] sw_ahci sw_ahci.0: forcing PORTS_IMPL to 0x1
<6>sw_ahci sw_ahci.0: AHCI 0001.0100 32 slots 1 ports 3 Gbps 0x1 impl platform mode
[    1.553972] sw_ahci sw_ahci.0: AHCI 0001.0100 32 slots 1 ports 3 Gbps 0x1 impl platform mode
<6>sw_ahci sw_ahci.0: flags: ncq sntf pm led clo only pio slum part ccc
[    1.568849] sw_ahci sw_ahci.0: flags: ncq sntf pm led clo only pio slum part ccc
<6>scsi0 : sw_ahci_platform
[    1.579635] scsi0 : sw_ahci_platform
<6>ata1: SATA max UDMA/133 mmio [mem 0x01c18000-0x01c18fff] port 0x100 irq 88
[    1.590415] ata1: SATA max UDMA/133 mmio [mem 0x01c18000-0x01c18fff] port 0x100 irq 88
[...]

Any hints what's going wrong here on warm reboot and how to work around this issue?

TIA
   Heinz