I have finally rid myself of this problem. I’m back on Linux firmware 5.9.x and Roon still has a problem with losing connection to my nas, but — this is only with cifs/smb. I finally figured out how to connect to my Synology DiskStation using nfs (see https://www.synology.com/en-us/knowledgebase/DSM/tutorial/File_Sharing/How_to_access_files_on_Synology_NAS_within_the_local_network_NFS ). Once I did this and pointed Roon to the nfs mount of my music folder, all my problems with Build 667 disappeared.
Those who are interested in doing this might want to check out the thread https://community.roonlabs.com/t/roon-stuck-on-scanning-now/124950. I would like to thank @Darryl_Caillouet and @mavmcl who first offered this solution to the problems caused by Build 667.
As they point out in the thread quoted above, it appears that Build 667 has introduced problems with cifs/smb. I hope that these will be fixed in the next build.