Update Manager - font problem - weird symbols

[AJSlye]

I've seen this many, many times over the years. That's a graphics driver and/or compositor issue. In many cases the remedy was different depending on the GPU, drivers and settings used. All you can do is try different combinations of settings until you find one that works.

Note: If this is on an Intel GPU, you could try switching from SNA to UXA for the xorg driver, this has been known to work in almost all cases when the SNA driver has become buggy.