This issue applies to users who select Roon Core as their designated Music Service to run on Euphony.

When you for the first time select to have scheduled backups for Roon in Roon Client interface, Roon will offer you a folder '/root' as the default folder. Don't accept this!
Go back by selecting '/' either in the side panel or on the 'Selected folder' path at the top and select '/data' folder as your backup folder.

Folder '/root' is located on Euphony System partition and this partition only has 3.4GB capacity with half of that already taken. 
Although the default setting is for Roon to keep 10 backups (99 max) and tries to assure you that "they shouldn't consume a lot of disk storage" Roon will not pay any attention to available free space on the partition where the backup folder resides and will happily consume all space until 100% is spent. 

This already happened to several Euphony customers: they chose the default backup location: '/root' and after a month or two all space on system partition was spent. Their systems started behaving erratically up until the point where they had to reinstall everything (and even lose those Roon backups!).
We don't have detailed information on how many backup copies those users chose to have and how big was individual backup (it probably depends a lot on the size of local library and/or usage of Tidal/Qobuz in Roon) but this is an issue we cannot guard users from as this is entirely in the domain of Roon and your backup location choice. 

While '/root' resides in a system partition with fixed 3.4GB capacity, '/data' folder is actually your data partition (we call it: Euphony Drive) where all your copied music, settings, database and cache resides. Data partition occupies all the capacity of your Euphony installation disk minus the 7.5GB that was needed for system and boot partitions. 

In conclusion: don't accept default folder '/root' and choose '/data' instead. 

Roon backup dialog after the correct selection (all default values except 'Backup Location'):