xwb_crossbar asserts when 1 or 2-bit address space is configured. It is due to the following if: https://ohwr.org/project/general-cores/blob/master/modules/wishbone/wb_crossbar/xwb_crossbar.vhd#L109

This assert makes sense if the bus uses byte granularity, but afaik WB supports word granularity as well (that I'm using).

Moreover setting c_wishbone_data_width to 8 will cause constant "align" to have a range (-1 downto 0) https://ohwr.org/project/general-cores/blob/master/modules/wishbone/wb_crossbar/xwb_crossbar.vhd#L83