Changed control panel security settings...and got "Error: This form has expired. Please refresh and try again."

In EE 3.4.6, relatively new installation. I went into Settings and changed a setting or two (I forget which ones, but most, I left alone. One thing I remember switching in the overall settings: I told EE not to save my templates as files.) Then I just saw this:

Error: This form has expired. Please refresh and try again.

I tried using a different browser (Chrome). I tried using ANOTHER different browser (IE.) I tried FTPing into the cache and manually clearing it all (since I couldn’t do it through the control panel) but it was just empty.

I’d really like to get back into my site and start the job of rebuilding it, but…I can’t.

Help??????