My last try of bisecting the error. I'm pretty sure I've got the correct commit
this time. I first ran 4 Weeks on the 4.9-rc1 without the error ever appearing,
upgraded to one or two release candidates higher, rebooted and instantly had
the error. I then did the bisecting in this window, waiting 3-4 Weeks for the
error to appear per commit, before flagging the kernel as ok.

Git then tells me, that 5f7f8f6edbf860abf18149a64be036d4be5e2993 is the first
bad commit.
I will attach the new bisect log. I don't know if the error might already have
been fixed, since I've been months on this ancient version. But I will try that
in the following weeks.