Q: "Outlook Data File cannot be accessed" after sync with iPhone 6

I am using Outlook 2013 (Desktop) on Windows 10.  Everything was fine before I set up iTunes 12 to sync the Outlook Contacts and Calendar with iPhone 6 Plus (iOS 9.3.4).  After the first sync, then Outlook on my PC cannot send or receive emails anymore, and it keeps giving the error message "0X800401F: Outlook Data File cannot be accessed".  After that ...

1.  I checked the Outlook profile, everything there seems to be fine, except I don't need to use the profile password to open it.

2.  I went into the POP and IMAP Account Settings and did a "Test Account Settings", and it went through fine with a "Congratulations! All tests completed successfully."  But I am still getting "Outlook Data File cannot be accessed" when i try to do Send/Receive in the main Outlook menu.

3.  I moved the Outlook profile (the .pst file) to another file location, reset where the email should go, and still the same error message.

4.  I logged into my email account by using webmail, my email account and password are good and no problem at all.

It seems to me the somehow after iPhone accessed my Outlook, it took out the profile password and "lock the profile".  I am able to see the Contacts and Calendars and emails, but I cannot send or receive emails into the pst file.

Actually last week I tried using iCloud to sync the Contacts and Calendar, the same thing happened.

What can I do to fix the problem?  My preference is to use my PC and not iCloud to sync.

Thanks for helping!