Third paragraph, with a quote directly after it:

An unnamed source told RIA Novosti the person responsible for drilling the hole filled it with glue instead of reporting the mistake.

"The glue dried and was squeezed out, opening the hole," the source told the media outlet.

It's fun that the astronauts did the proper/final fix with epoxy, which to most people is of course also a glue. So perhaps the worker used (or mis-used) the wrong glue to fix this, rather than tried to cover up the mistake? Of course it sounds reasonable that it should have been reported, which didn't happen, so so the working environment seems improvable.