Occasional Kernel Panic

Can you try to perform disk test? here is the link for the tools -http://www.readynas.com/kb/faq/hardware/how_can_i_verify_that_my_disk_is_bad

Also contact tech support and open a troubleshooting case.

I didn't try that yet but I will. I guess that it will take me four days or so. Removing disks from readynas -> connect to windows machine and test -> back to readynas and wait for resync. (repeat four times).
I'll let you know the outcome of this test.

Meanwhile, are there tests that can be run from the NV+?

I've run the long disk test on all four disks, without any problems. So the disks are okay.

Are there loggings available on the NV+ that keep track the events during the booting process?

A firmware reinstall wouldn't hurt - if the root cause is a firmware issue (eg corrupt file) then that could be a simple & quick way to overcome without affecting your data. See FAQ for instructions

Also consider temporarily disabling any addons or non-required services - this might help narrow down the cause if the issue "disappears" so that you can start re-enabling the addons one at a time

Else, you need to download your logs and search for clues which maybe you've done given you are stating the NAS experiences a Kernel Panic - or are you sure of that given this statement?

R12 wrote:
Are there loggings available on the NV+ that keep track the events during the booting process?

Same logs record the boot process

I have reinstalled the firmware and also reviewed the add-ons.
Since then I have rebooted about three times without problems. Let's hope it stays that way!

Thanks for your help

After a three week period of daily system starts didn't give any problems. So I think the issue has been solved!!