Hello Roman,
Invalid byte at start of character: 0x99 [Condition of type TRIVIAL-UTF-8:UTF-8-DECODING-ERROR]
This seems to suggest that the text in the record is not actually ASCII, but some other 8-bit encoding. 0x7f is the highest character code ASCII text can contain. What does 'psql -l' show? Actually, the way I understood it is that if you set a UNICODE/UTF-8 encoding for the connection, which Postmodern does, it should translate any text into that encoding before sending it... but that would mean the error you're seeing could never occur. I've never seen it happen before... switching your database to UNICODE encoding is bound to fix it, or you can try setting Postmodern to use 8-bit encoding (with the 0x7f-0xff characters being interpreted in whichever way your Lisp implementation feels like, most likely latin-1) by changing the *unicode* parameter in the cl-postgres.asd file, and re-compiling the library.
Best, Marijn