Trying this is foobar and documented in:
Adding network shares is not possible for apps with DSM 7 and beyond. If you need too, mount the network shares through File Station.
And more detailed information in this thread:
Please click on the blue thread title to go there and read also the posts following the one I linked.
Possibly because you loaded a backup from your old Core. Feel free to change the Core name in Roon|Settings|General
.