Same problem, same german Board
SlimBean7,8 with Yank 1.6e and DLIB modem.
Another modem doesn´t change the problem. It shouldn't be a problem with (mobile) data, because the airplanemode does not interrupt the problem like a restart and there isn't a bigger traffic after 24hours.
As told, we tried a lot with recievers and event services from systemapps etc.
temporary, a restart ist the only solution.