Manfred,

I don't know if your source contains all the interrupt handlers for the interrupts you enable, but that only as a side note. The main problem your code has, is, that it waits for TI to be cleared ( jb TI, loop), that is, stops on a set TI, what won't be the case immediately after writing to SBUF. You will want to turn that into "jnb TI, loop" and everything should be fine.

Another side note: I only initialize the things concerning serial data transmission once, not in a main loop, like you do.

hth
-uwe