It seems the 2 layers weren't glued properly, so resin moved into that capillary. Leading into 2 problems:

First, detach was not to easy. Some of the screws didn't come of easy. Most could be detached with a bit of force until the thread moved inside the resin.  A better choice would be stainless steel screws which stick a lot less to the resin.

I break of one head of a screw (bolt), which I could remove with just more force and pliers.

But the bigger problem is the resin "going sideways" between the wooden layers generates a cavity in the resin, having only one letter without a small or big air-bubble inside (the "L" at about the 8/9 position):

I did try to fix it by getting resin into the bubbles from above. So I might know the success of that in 24h (curing time of the resin.

Ideas to prevent this: 

better, tighter method to glue the 2 layers together before applying the resin.

Do not use epoxy with a long curing time, but resin for 3D printers, use acrylic as the bottom layer and cure every cavity right after the injection of the resin by applying UV light from underneath.