I have an Outlook account and when I tick the Tasks/To-do’s as completed I get this error and although the task remains completed in eM Client it is not reflected in Outlook.
I had the same error when I updated some contacts and they would not synch with Outlook. I deleted the contacts in Outlook and it worked fine.
I have removed and reinstalled the Outlook account in eMClient twice and it temporarily fixed the problem, although it comes back, usually with a different scource.
Hi Gary, Yes I set it up as Exchange and here is a picture. Also, if I edit the item from the error message to ‘task not completed’ and then go into Microsoft To-do and complete it, that completion is synchronised with eM Client.
It doesn’t work from eM Client to Outlook–it does work from Outlook to eMclient. 
Well, that account is setup as IMAP not Exchange. When you add an account, choose Mail > Exchange. The implementation is slightly different because it uses EWS rather than AirSync. Give it a test and see if it works better for you. You will still have access to your same online data.
The first time I tried the above it installed a MAPI account again, the second time I managed to get Exchange by putting the server settings in from Outlook. Although I cannot see the account in eM Client. 
Add a new account. Click on Mail > Exchange (as in my screenshot above).
The next screen asks for your details. Enter your email address in both the email address and username fields. Enter your password and click Next.
That is about all you need to do except confirm your name and email address, and verify you want mail, contacts and calendar.
By jove that seems to have done it! Thank you. 
Hopefully the tasks and contacts will sync better.
I have tried to synch the tasks both ways and it works beautifully, thank you for your help.
Hello, I tried the same for linking my microsoft account to emclient but it does not work. After having filled in my password I get the message : “We’re unable to complete your request… unauthorized client. The client does not exist or is not enabled for consumers…” The message adds some instructions for application developers, which I am not. My Gmail account works fine. For Outlook the situation is frustrating.
Hope that someone has a tip.
Best regards
Jos Tans