Admin pages redirecting to site homepage

On more than one of my sites, I’m getting redirected from the admin dashboard back to the site’s homepage. It seems to be sporadic, not always when clicking on the same admin nav item. Sometimes it’s when trying to go to the plugins page, sometimes when trying to update a page content, etc. Doesn’t seem to be any specific links where it’s happening.

The sites are using different themes, the only common plugin is askimet, and two of the sites have very few plugins. I’d turn off the ones that are there if I could stay on the plugins page long enough to do so! ??

The only similar issue I’ve found listed here or elsewhere was about a year old and seemed to be related to a security plugin. Any ideas what might be going on?