I'm having the same problem. Does anyone know what's happening or how to fix this? Except I have had private mode setup since I got the phone. This has only started since the new update. Please help!!
Same exact problem here. Set up private mode on an S5 using fingerprint scan. Fingerprint scanner broke, was locked out of private mode. FINALLY Android released an update that would allow me to use my alternate password and it worked fine. Then I upgraded to a Galaxy s7 Active, everything worked fine. Upgraded to Android 7, and now again when I try to enable private mode I get the "Failed to turn on Private Mode. Try again."
The only answers I can find about it are old ones suggesting to swap from ART runtime to Dalvik. But as far as I can tell Dalvik has been removed as an option for a while now... I tried turning on OEM unlocks, seemed to do nothing... And I tried the most obvious procedure, restarting the phone. I have no idea what else to try.
Any help would be greatly appreciated. Thanks in advance.