Suddenly shutdown when write down to MP4.

Hello,

I using iMovie with my Mac mini.

Since 3 weeks ago, I met troubles in it.

After create movie, try to write it down to MP4 file, Add name, Choose destitation folder and press "OK".

After that, iMovie will shut down suddenly.

Then it shows error message "iMovie had shut down by unexpected reasons(Sorry I don't know it is correct or not, because I using Japanese version) and show error log.

I tried to restart again, remove old event and libraries, re-boot computer, re-install iMovie, but remains same.

I try remember that any suspicious event before this trouble was occurred, I think it was occurred after received update of Big Sur.

I don't know it was concerned to this trouble or not, but not any other change was performed.

Please help.