As we've just found out, because of a very old and stupid bug, a piece of code responsible for keeping the JB alive had a crippling bug that subtly broke stuff on FW 5.10.x.
In order to get things working again, you'll need to update to the very latest
JB hotfix (either before or after the update), and then, just to be safe, as far as my own stuff is concerned, to use packages from the
snapshots thread when reinstalling stuff after the update.
And that's it, crisis averted

.