I am having the same problem mounting my Synology NAS after upgrading my PI. It was working fine with the old PI and I made notes of the settings and the current ones look the same. These are the results of my checks.
Using PuTTY to do a showmount request.
volumio@volumiopnb:~$ sudo showmount -e 192.168.0.249
Export list for 192.168.0.249:
Screenshot of V’ setting page attached. Unless I am getting data blind, they look correct!
I have no record of setting a user and password but tried my admin details for the NAS but it didn’t solve the problem.