Unable to access Appearance>Customize

For months I’ve been unable to reliably access WordPress’s “Appearance > Customize” from any browser, including Chrome, Safari, and Brave. It happens whether my plugins were active or deactivated. Everything is updated in my WordPress.

The most recent browser error I’m getting is this:
“The HTTP response body is transferred with Chunked-Encoding, but the terminating zero-length chunk was never sent when the connection is closed.”

Anyone heard of this problem before?