ACT led turns eventually to stable ON

Running Volumio 1.4 and Hifiberry DAC for several weeks now. Boot is ok, mpd serves nicely net radio and files from USB-SSD (separate powered USB hub for SSD, main RPi power is separate 5V 2A). Works for hours. Then after some hours I’ll stop the play. At some point more hours later when RPi should be doing nothing, just idling, I check the machine (because it is not responding) and it shows the ACT led glowing constantly, not blinking. Volumio is not accepting SSH connection or any other connection. I can only reset or power off/on and after this Volumio is responsive again and everything works. For an unpredictable amount of time.

Any tips what I should be looking/checking? Any specific log or something else?

In daily use I only have WIFI connection to the box but I can connect a display and kbd if need be.