All…
Significantly narrowing the issue:
Downloaded 2 Volumio IMG files, (latest and an older one)
Burned on flash…
BOTH work fine, right out of the box, nicely initializing the BOSS-DAC.
The main difference is:
I did NOT activate:
- The touchscreen plugin
- The power switch plugin
Just use it with a win-laptop remotely.
Next, I tried installing the plugins, to see where it goes wrong…

First one is a hit: the touchscreen plugin…very surprising.
It display’s a warning that it is not recommended with PI 3B+, but… it did work perfectly before…
Next, it gets always stuck on line “21 OTHER” with fonts or so…
Need to force it to reboot to get out of this situation.
After reboot, the GUI is NOT displayed, even when the plugin is set active.
(Only the Volumio login, which still perfectly works with external keyboard)
Important remark
:
The install says V1.00, while the plugin is V1.01 (maybe it installs an old version from repository??)
Puzzled for a while, why now the display suddenly does not work…
Then the “AHA”effect… : 
Initially, I run the PI & display without the BOSS-DAC… (as a temporary solution to play,I used the pi output)
At that time, I installed display, remote pi, auto start plugin, and this worked fine…
After that, my BOSS-DAC arrived, and I plugged it in…
And problems started…
I recall very often a message on the screen after reboot, that chromium asks for “restore”…
A clear indication that the nice shutdown I perform is not executed correctly 
This is quite a pity, as this native 7” PI display is a very important & positive contribution to the “WAF-factor”
(The “Wife Acceptance Factor”, for all audio equipment in the living room)
@ Volumio:
From my electronic engineering background, it cant be too hard to fix this natively supported PI display?
As you already display the native Volumio login on it by default.
No need to be fancy or so, just the basic GUI & tap… to pause and play it, search for music
It seems a conflict on I2C, and or something that chromium kills on shutdown.
(unsure what this chromium is doing anyway for the display/audio player)
Be aware, that it DID work flawlessly for a while with display, if the display plugin is installed first, THEN the DAC installation.
Only after a few reboots, chromium restore messages, the ALSA trouble starts…
Hope this analysis helps to fix it for others too.
Regards from Taiwan.