Hi xpalm, it's really helpful. It solved my issue. Thanks a looooot!
Product:DS3617xs
Version:DSM 6.1.5-15266 Update 1 (yes, the least DSM version)
bootloader:DS3617xs 6.1 Jun's Mod V1.02b
But. I noticed, in my case, each time, when I upload photo (via DS Photo or Moments), trigger the thumbview process backgrounds, then there will be big chance to make me encounter this issue again..., anyway I can recover it by the simple WorkAround you provided.
I'm still wonder what's the root cause for this issue. if anyone suffered similar issue like me?
