The problems occur with another two PCs. The cable isn't the problem because I connected the programmer direct with one of the PCs (without cable) and it behaved similarly.

One frequently bug is the following: when the byte 07h (0000 0101) is sent, it mutates to 77h (0101 0101). Most errors occures with bytes, where the first four bits are zero. But estimated 95 per cent of all corrupt bytes are 07h and become 77h. That's strange.
Within 2kB about 10 mistakes can be found.