Ok, currently there is no easy way how to diagnose such issue. This will improve in v. 8.2 that will include logging for cloud storage protocols. However if you can give us your email address or send it to me (burger @ emclient.com - please remove the spaces, this is a spam protection) we’ll look in our system whether we’ve received some error message for that account and let you know if we find something.
One thing with attaching files from One Drive as a “Link” is that you have to “Allow One Drive Authorization” to EMC when you click “Yes” or you cannot attach linked files. See example below.
Type in your Microsoft login email address & click next.
Click "Yes " or “Enter your password” depending on your current security setup in One Drive, to let this app access your info and the file attachment link to then appear in the eM Client message.
If you have any problems and the One Drive authorization window doesn’t appear, go to the following Microsoft Account Website and remove any old eM Client authorizations and try again.