ICX7250 Stack Offset Problem

[cankaya_univers]

Hello,

During interactive stacking, I get this error:

You selected 8 unit(s): #1: ID=2, #2: ID=3, #3: ID=4, #4: ID=5, #5: ID=6, #6: ID=7, #7: ID=8, #8: ID=9,
T=2h5m50.7: Error! stack interactive-setup option 2: link 9/2/1 to 1/2/4 has different offset.

On ports 9/2/1, 9/2/2 and 1/2/3, 1/2/4 there are fiber cables connected between them, rest of the cables between switches are DAC cable.

What is this? what is offset?