The latest error I have in the log dates back to 3 days ago. So... I have nothing in the log.
Maybe I will open a bug, so we can see if others experience the same issue. I exhausted my ideas.

On January 25, 2016 3:49:49 PM CET, "A.L.E.C" <alec@alec.pl> wrote:
On 01/25/2016 03:38 PM, devotedmystic . wrote:
 Also managesieve produces a blank page. Other plugins seem to work fine.

Maybe something with classes autoloader. Anyway, in such case (blank
page) you'll see an error in log. So, take a look at your error logging
configuration.