Link to the same problem: https://xpenology.com/forum/topic/8066-dsm-613-15152-update-5/?page=2&tab=comments#comment-78231 but on U5 and 1 post below - on U4 even. So probably not related to updates, or at least not to lates updates.
I had same problem a week ago on U7, but was thinking its HW issue.
Solution to migrate will probably work, but its better to go to root cause, otherwise it might happen again and again...