Xbindkeys conflicts with someone, how to find and fix?

Question

ubuntu

Greetings!
I'm trying to redefine the back and forth keys on the mouse, put xbindkeys
writes:

*** Warning ***
Please verify that there is not another program running
which captures one of the keys captured by xbindkeys.
It seems that there is a conflict, and xbindkeys can't
grab all the keys defined in its configuration file.

O_o xbindkeys -k does not react to mouse buttons, to the keyboard - everything is ok.
xev button sees:

ButtonRelease event, serial 36, synthetic NO, window 0x1400001,
root 0x268, subw 0x0, time 4543315, (96.63), root:(2341.704),
state 0x2010, button 9, same_screen YES

did killall xbindkeys - no change.
Didn't set anything else.
What can be and how to look for, with whom it conflicts?