If I am remembering correctly, I think you are located in Greece? Assuming yes and, if you have an exchange based VDSL2 service, then the dip (or notch) seen in the DS1 band cannot be down-stream power back-off (DSPBO), with which we are familiar, for cabinet based VDSL2 service in the UK. Hence, logically, it must be the result of cross-talk with other xDSL services. (ADSL2+ will use sub-carriers 6 to 511.)
The cause of the bit swapping pattern that you are seeing will be down to something switched being on for a six hour period, possibly time-clock controlled. The fact that you are seeing the bit-swapping, without any errors being recorded, shows that the technique is working well and is keeping your service stable.