I’m sorry that mounting is causing problems. Recently, many here in the forum have had different problems with it – mostly due to the new DSM version 7.0.
Maybe their experiences will help you. Here are the latest threads on the same topic (some are in german though):
In the two German threads, nfs was used for mounting. The solution to the problem was to specify the path correctly in this form:
IP-Address://Path (Path example: volume1/Music/)
Sometimes it also helped to change the NFS version in the Diskstation settings and share the music files with „Everyone“. They have also left out the other options (user, password etc) for NFS sharing.
For more information, I recommend you also read our wiki article on mounting: https://www.max2play.com/en/wiki/mounting-an-external-drive/