S4148T - way to see spanning tree topology changes

Hi there,

my edge/workgroup switches all show a spanning tree topology change in their logging via:

show logging

All leads to the port channels to the servercore switch the s4148t but i dont see any hint in the log via:

show logging logfile

loglevel is on debug.

do i make something wrong or is there another way to figure out what caused the spanning tree topolgy change?

any hint is welcome