Roon Server Machine
NUC I5 5250 16GB
Roon Server 2.0 1357
Networking Gear & Setup Details
Wired connect to Google Home Mesh - Zysel VDSL modem/router (not in bridge mode) - Double NAT for ARC
Connected Audio Devices
Roon Remote (2.0 - 1357) on Android & Mac (plus other devices not tested during this problem)
Number of Tracks in Library
17500 tracks
Description of Issue
Library is housed on a NAS (Synology DS220+) and access via Roon web access (I presume SMB). This morning I updated the NAS to DSM 7.2.1-62095 Update 3.
For some reason Roon decided it needed to rescan my library. It then reported rescan complete and updated but there was a circular spinner on the home page. The library did appear fine and clicking on albums would show tracks etc. Any connection to Roon was SLOW and ARC would not connect. Killing Roon Remote App and restarting did not fix the problem and Room Remote on MAC exhibited same behaviour. After about 1 hour I rebooted the Roon Server NUC machine.
This fixed the Roon problem, but ARC was still hanging on the startup splash screen. Killed the ARC app and restarted but still hanging on start. Uninstalled ARC app and reinstalled**, went through ID and connect to server process fine. ARC works fine (and seems faster/snappier)
So all problems fixed. Am filing this as just a bug report. Wonder why updating NAS OS prompted NUC to rescan - the SMB address of the files was unaltered. Wonder why Roon library scan seemed to go into a loop after apparently completing, consuming significant resources. I don’t know Roon processes enough to try ‘top’ and ‘ps’ etc to find culprits. Wonder why ARC would hang on connect (if it was having problems why not fail?) and so need reinstall.
Anyway no problem apart from higher blood pressure 
** ARC install has a usability error - when signing in it insists you verify that ARC was downloaded from Roon only and nowhere else. Yet it you visit Roon download page it states that ARC should be downloaded from Play Store/App Store (although further down there is a Downloads section that suggests both of those sources but also has an Android APK). This is a mess and should be tidied.