I've been wrestling with a problem for a long time and I think I might have found the cause. I wanted to share with the group in case you encounter it too.
Read this article for a description of the problem
I had the same problem where only SOME Mac users were having this problem, but there didn't seem to be a commonality to explain the problem. The problem also survived through several Lync updates.
The solutions offered by MS kinda sorta work but I discovered that completely removing any and all Lync files and the program then reinstalling is unnecessary. This morning I made a script that completely removes all Lync files from ~/ as well as completely deletes ~/Library/Keychains/ When I ran that on a user's Mac it worked! We did file transfers to and fro with his Mac and my Mac. All was well.... until an hour later. The problem returned on his Mac. He notified me by email because I wasn't at my desk but when I messaged him in Lync to followup with him, I noticed something. His font. The font he changed for his messages was not the default Verdana 12 Black. It was some scripty, 14 point, orange custom setup. That's when it hit me. Doing the sledgehammer purge of all Lync files, this also purged his preferences that contained his custom font setup. When we did the after-purge testing, it was back to the default. When he got back to his desk, he immediately customized his settings but didn't try sending a file for another hour. When he messaged me with his newly custom fonts, I had him change it back to Verdana 12 black and it worked again!
After some tinkering and experimenting, we discovered that he can change his fonts just a little bit and it will still work. The wilder the changes, the greater chance that the file transfers would fail. I looked at my trouble tickets and all the users who complained about this problem all have wild custom font settings. So far setting back to default fonts (without using the sledgehammer) has fixed it. I don't know if it is specific fonts, specific sizes or colors or just odd combinations of the three but it is DEFINITELY caused by the font settings.
I don't have a hotline to the Microsoft Lync developers nor do I have the patience to create a Microsoft account so I can try to navigate their support site and find where to report my findings. So hopefully someone from MS will see this post and relay the info to the developers. But more importantly, hopefully you, my fellow Mac admins, will find this post in your search for a solution and you can find it useful.
config: OSX 10.9.5 through 10.10.5
MacBook Pros and Airs of various specifications
Lync 14.0.x through 14.3.3
I'm glad you were able to independently confirm my findings! I was so excited when my user messaged me with his gaudy font setting and I made the connection. If it was a more subtle font change, I might not have ever made the connection. While Microsoft's solution of deleting all the files was technically correct, it didn't address the cause of the problem. Now that MS is working on Skype for Business for Mac, for all we know this problem may never come up again, but we all know developers like to re-use code when they can. Thanks for running it up your support chain.