> sqlobject.dburi="firebird://user:password@localhost
> :3050/pathToTheDatabase/automaty.fdb?charset=utf8"

Finally, it looks I was focused on the column values to much, when hunting mentioned UnicodeDecodeError.
You can find the solution here:

The names list generated in:
names = [self.sqlmeta.columns[v[0]].dbName for v in setters]
(_SO_finishCreate method in main module)

contains unicode type string elements (column names) for the columns, that are not manually defined in the model.

because of that:
        return ("INSERT INTO %s (%s) VALUES (%s)" %
                (table, ', '.join(names),
                 ', '.join([self.sqlrepr(v) for v in values])))

UnicodeDecodeError "'ascii' codec can't decode byte 0xc4 in position 95: ordinal not in range(128)"

Yes, it looks strange, but:
  1. unicode type string is generated from the  ', '.join(names), when there is at least one element in the list containing unicode type string
  2. an unicode type string is generated from the  "INSERT INTO %s (%s) VALUES (%s)" when the first part of the string formating is executed
  3. when there is an encoded string in the values, it is not possible to execute the second part of the string formating and an exception ocures

to understand it, try to experiment with the following code:

table = "pokladna"
myJoinedNames = u'id, autorizovano, vydal_osoby_id, mena_id, vydal_automaty_id, platny_zaznam, t_stamp, castka_czk, prijal_osoby_id, vycetky_od_automatu_id, prodejni_mista_id, prijal_automaty_id, pohyby_zbozi_id, castka, id_hash_vydal, poznamka, ucel_id, id_hash_prijal'
myJoinedValues = "3219, 0, 3, 1, NULL, 1, '2012-02-26 00:41:31', 2000.0, 16, NULL, NULL, NULL, NULL, 2000.0, 3, '\xc4\x9b\xc5\xa1\xc4\x8d\xc5\x99', 3, NULL"

print "INSERT INTO %s (%s) VALUES (%s)" %  (table, myJoinedNames, myJoinedValues)

As an "short and dirty" solution I have defined all columns in the affected table manually. 

I am not sure, if above mentioned is not  caused because of I have rewritten columnsFromSchema and guessClass methods in the firebirdconnection module.

I am pretty sure, Oleg will come with an other, elegant solution for this problem.