Figured out the problem.
Since MS changed a lot of things regarding sharing in Windows 10 you are no longer able to share locations between Windows 10 devices without actively performing authentication (i.e. even the most unsecured form of sharing folders between devices requires you to type in ‘guest’ as username. Since SR doesn’t support providing credentials this lead to the service not being able to authenticate.).
Since I’m not working with a domain situation I had to figure out a way to authenticate my SR-running installation to my file-serving installation. With homegroups also being out the door in W10 1803 there aren’t many options left.
The solution I found to work was to change my device local sign into ‘Microsoft Account sign-in’ on both devices. With the same account existing on both devices I was able to grant share permission to this user and consume that permission from SR by running the SR service under that same account.
Use “Microsoft Account sign-in” on all devices if you are running a modern installations of Windows 10 to circumvent the need to provide credentials when connecting to a UNC path.
Obviously this only applies when using stand-alone W10 devices; when using a NAS or domain based file services this solution does not apply.