Some admin pages produce white screen of death

I am at my wit’s end trying to figure out why some of my admin pages produce a white screen of death…..
ie. clearing cache produces blank page
updating or changing blocks produces blank page
changing theme settings produces blank page
and more…

There are no error entries in my error log…for these…
I have had to re-upload drupal 8 core files to get rid of some error messages but this doesn’t fix the WSOD pages….

Anyone have any suggestions?
my client’s site is

thanks so much

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