I am leaving this not in hopes it help someone else. HOST computer: Dell Optimus desktop computer, Windows 10 build 21H2. CCleaner run, MalwareBytes run – no infections. CHKDSK C: /F and restart. HOST LOGS show no errors. VIEWER computer: Acer Laptop, Windows 10 build 21H2. CCleaner run, MalwareBytes run – no infections. CHKDSK C: /F and restart. VIEWER LOGS show no errors. Router: Turned off for five minutes to reset route table. VIEWER Error: ‘Cannot Open Directory’ in File Transfer function. Solution: When on home network using NO-IP ddns; gets the error. When on home network using router-reserved HOST ip address; NO ERROR. FIX: Created two VIEWER desktop shortcuts. One shortcut to HOST using ip address when on home network. Create second VIEWER desktop shortcut to HOST using ddns on outside house network.