There seems to be a missing mapping for monospace fonts (the mappings are all present and correct for Serbian in regular Ubuntu font).
In regular style and weight (UbuntuBetaMono-R-R21.ttf), it's missing a substitution for lowercase "б": afii10066 to afii10066.local.
In italic (UbuntuBetaMono-I-R21.ttf), it's missing all five substitions for б, г, д, п, т.
The glyphs are all there. I haven't checked the Bold (there doesn't seem to be Bold-Italic), but I assume it has the same problem as regular.
There seems to be a missing mapping for monospace fonts (the mappings are all present and correct for Serbian in regular Ubuntu font).
In regular style and weight (UbuntuBetaMono -R-R21. ttf), it's missing a substitution for lowercase "б": afii10066 to afii10066.local. -I-R21. ttf), it's missing all five substitions for б, г, д, п, т.
In italic (UbuntuBetaMono
The glyphs are all there. I haven't checked the Bold (there doesn't seem to be Bold-Italic), but I assume it has the same problem as regular.