I use these commands whenever I have problems with the log facility and I need to reset it.
Set the maximum size of the memory buffer log, in bytes. (Default 65536)
config log memory global-setting set max-size 2048000
Clear all logs
execute log delete-all
Memory log settings
config log memory setting set diskfull overwrite set status enable end
Send log report to gui
config log setting set gui-location memory
Format the log disk
First change the log location in the gui to something else (like memory) and than go to cli and run (the fortigate will restart after that) use with caution!.
execute formatlogdisk