A zabbix notification was not sent because of special characters in the "units" field of the item.

I added a screenshot of the item and the audit log.

Removing the "°" character of the item solved the issue.

As i can remember zabbix releases prior 1.8.15 do not have problems with that kind of special characters.
This was really bad for us, because we got no information that our datacenter had temperature problems.

Probably this issue is also related to ZBX-3133.
It might be a good idea to have the possibility to shorten/mangle messages or to send a fallback message (something like "Unable to deliver the message, see zabbix gui....")
if sending of a certain message failed.