Cannot Delete Sheets (Again!)

I had this problem before: when I try to delete sheets, I get an internal error:

• Internal Error - An unexpected error occurred and the operation could not be completed.

I had tried the trick of putting unwanted sheets in a 'junk' dashboard and then deleting the dashboard, but it did not work.

Then I tried it again and it did!

Now it doesn't again

I am signed in, so it's not the type of connection problem I've had when I try to save and find I've been logged out.

This is very frustrating! Help! Please!