I’ve found the solution. It seems that if you follow this guide: https://www.ibm.com/developerworks/library/se-oauthjavapt3/index.html you will see that when you exchange the access code for the access token(/realms/{realm-name}/protocol/openid-connect/token) you need to add the same redirect_uri that you used when requesting the access code. It works for me now.
Hope it helps,
Adrian