It definitely sounds like something is going to sleep, or going into low power mode. If restarting the DAC doesn't fix the issue, and rebooting the PC is the only thing that does, and it has always happened on this PC, then it must be something to do with the Power settings in Windows on the PC or in the BIOS.
First though, please confirm where the TOSLink connection comes from. Is it the Motherboard audio, or do you have a sound card installed, or some other hardware?
Is USB involved anywhere in the digital audio out, perhaps with a USB to TOSLink converter?
If the problem happens after two hours, or some time you can narrow down, does that time match one of the Advanced Power & Sleep settings in Windows?
There isn't a specific setting for audio on the motherboard, but there is for USB. Try disabling "USB selective suspend", which may be related on your motherboard.
Is there a specific audio driver installed for audio on the motherboard, or whatever outputs via TOSLink? Any sleep or suspend settings in there?
Does other audio work when this happens in MC? i.e. Can you play the test audio tones in Windows Sound settings (device properties)?
If you can play audio out through Windows Sound settings, have you tested in MC after that works? Perhaps that would wake the output.
If you have or can get another media player (i.e. Foobar2000 with WASAPI plugin), then when this happens, close MC and try to play back in the other application, does it work? If it does, does MC work after that?
Does audio work from a browser when this happens, and close MC? i.e. YouTube video plays audio?
The other area is BIOS, or UEFI, of the PC. Have you checked power settings in there? Anything that looks like it may power down the motherboard audio?
There is a small chance that leaving MC in the Paused state for a long time would cause issues, even if just audio is being played. I've never tested for that. It would be best if you stopped play rather than pause. You can set up MC to resume where you left off, even for audio files if you wish. MC27 has new functionality to make that easier, but it can be done in MC26.
27.0.4 (8/22/2020)
11. NEW: Added a new search to options to allow setting a grouping of files to automatically apply a bookmark (so if "Use Bookmarking" is set to default, it turns on for files that match the search).
That is probably enough to think about for now. Let us know how you go.