In the logs the error is “access denied by server while mounting” and this could be a permission problem in the nfs config on the Synology as @fzinken now already claimed several times.
I cannot reach my Synology nas for the next 3 weeks but in the following link you see the menu where you configure the ip/ip range of the clients which should get access to a specific share: Unable to connect / access NAS anymore - #5 by JimKnopf