I know there are other posts about this but cant find them (dont find GoG very helpful in searching forum posts). I deleted my save files and restarted this quest in order to try and get access to the chest. First time round the chest glowed in witcher sense and the stopped. Second time round the chest kept glowing even after i had picked the letter and the key up but then after a period of time, even though i was in the process of highlighting with witcher sense, it just stopped. Even when the chest was glowing you were unable to access it.

Posting this in case it helps the devs figure it out and hopefully so we can get a fix. Will leave the quest open for now