I agree that switching to the LBook firmware may solve your problems.
For some formats, including (I think) TXT you can add your own fonts that include the characters that are showing up as missing (?). If some TXTs are working and others not, then they may be using different character set encodings. This is difficult to allow for in TXT, since it has no metadata. The default encoding for TXT was probably switched when you selected Russian as the primary language, but there is more than one possible encoding.
For ePub, you can add your own fonts but only by adding extra CSS to each ePub, see
Fonts and Epub - What works on Sony, Works for Pocket Pro too!