@nebulon @girish I should have wait before posting the screenshot because you guys have lost focus on the cause lol ... I cannot tell why all of a sudden the admin UI appears like that in -windoze- (yeah, you're right it was the first thing I thought) in FF and Opera.
On mobile, I've got disconnected (kicked out) from my server but not yours, as soon as opened the Android client, and I have no more access to the admin from the mobile client so can't tell for now if the font issue would appear.
However this font-glitch does not appears on my Linux WS, meaning everything looks just fine but the ERROR RochetChat sends on an attempt to message someone and what SHOWS UP in the LOGS at the same time remain, whether on windoze, Linux or with Android client, as I explained at the top of my OP.
So yeah, clearly it seems like a glitch is -also-maybe- coming from the instance somehow. And of course I noticed that yours works well and so that renders me even more clueless.
The logs should tell someone something, they are on the server side?
Meanwhile I will try to install a new instance from my Linux WS and see what happens.