Could you try the latest version (16.10.12)? It won’t fix the problem but it could give different error message which would help identify the issue.
16.10.13, New error is “IOException: Error writing request”
p.s. Love the new PDF button. 
There is definitely something very strange going on. Could you install Manager on another computer in the same network to see if you get the same problem?
Just installed and tested on VM with Linux Mint 18 (on same host with problem) and it worked fine. Imported business and that worked fine.
Tested a dummy business on my normal desktop and got
“ObjectDisposedException: Cannot access a disposed object. Object name: ‘System.Net.Sockets.NetworkStream’.”
It used to work fine before a recent update.
However, since I can save the pdf locally, I can email that so don’t worry about spending any time on this if it is only my machine that is playing up. I can work around it.
Cheers
@Zark - I have a theory - could you try something and report back please
On your Linux Mint 18 installation run the following command in the terminal
sudo service network-manager restart
Then try and send your email and let us know what happens
Thanks for the suggestion.
For clarity, I have LM 17.3 as normal OS/host and LM 18 as the guest in Virtual Box. LM 18 (guest) is working fine, LM 17.3 has stopped working.
I tried the terminal command in both installations anyway, but the result is the same.
When I tried a custom SMTP server. the test email went out fine. Only when trying to email a report does the problem occur, which makes me think my system (at least the host OS) has an issue with communicating with the PDF generating web service.
I checked mono just in case, and I have 4.6.1 installed. I’m not aware of any changes to my system that might block that service.
Once @lubos is happy with the local pdf generation, I’m sure this issue will be resolved.