Raspi3 B+ external SSD USB freezes and hangs

For some time I've tried to get my Raspi3+ext SSD Setup working. Currently I do a complete Backup via dd towards a NAS drive. The Image sizes are rougly about 100 GB.
between 2GB und 100 GB the Backup always crashes.
The SSD ist connected directly to the USB Port without additional Powersupply.

So far I've had the following errors:

<pre class="brush: csharp">// Comment

watchdog: BUG: soft lockup - CPU#2 stuck fpr 23s: [kworker/2:0:3391]

....

watchdog: BUG: soft lockup - CPU#2 stuck fpr 23s: [ksoftirqd/1:18]

...

watchdog: BUG: soft lockup - CPU#2 stuck fpr 23s: [swapper/2:0]

....

rcu: $1-...0: (2ticks this >GP) idle=......02 softirq=12313/123123 fqs=18361

...

cpufrq: __target_index: failed to change cpu frequency: -22

....

watchdog: BUG: soft lockup - CPU#2 stuck for 22s! [(d-logind):5635]

....

rcu sched kthread starved for 1233 jiffies

RCU grace period k-thread stack dump

</pre>




So far I've tested the following:

External Power Supply for ssd
Differen connector for SSD
USB Stick instead SSD
Backup from SSD to USB Stick
Backup from SSD to SD Card
Backup from USB Stick to SD Card

Everytime I get some kind of Kernel error, cpufreq error aso.

Next Step ist to test Raspbian instead of ubuntu.
Till now, raspbian was able to do a successfull backup to sd card in the same pi.
Next Step ist Backup to Network NAS.
If that works as well I'll try my ssd with Raspbian an Backup to NAS.

Actually, the only solution was a Raspbian native install on SSD. Everything works as perfect since then. qed.