It looks like the SIO is expecting to get a clock that is 16x the actual bit rate here:

Z80SIO ":upd7201_new" Channel A : Clock Mode 16X

Are you sure the BRG is producing the right bitrate? Also remember that you need two call to, for instance, rxca to shift one bit into the SIO, as only every second is a raising flank, given that you only call it when the clock changes state.