Hello. I have same issue. But as far as I can see login and password are correct. It's calendar trying to sync. It's send to Exchange empty login and generates flood so logs are growing up.
Looks like this:
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 - 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 401 0 0 0
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 - 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 401 0 0 15
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 - 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 401 0 0 0
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 - 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 401 0 0 0
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 - 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 401 0 0 0
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 - 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 401 0 0 0
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 - 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 401 0 0 0
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 - 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 401 1 2148074254 0
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 - 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 401 0 0 0
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 - 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 401 0 0 0
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 - 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 401 0 0 0
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 - 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 401 1 2148074254 0
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 - 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 401 1 2148074254 0
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 - 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 401 1 2148074254 0
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 - 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 401 0 0 0
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 - 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 401 1 2148074254 0
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 - 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 401 0 0 0
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 - 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 401 1 2148074254 0
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 LOCAL\nectop 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 200 0 0 78
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 LOCAL\nectop 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 200 0 0 499
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 LOCAL\nectop 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 200 0 0 484
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 LOCAL\nectop 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 200 0 0 499
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 LOCAL\nectop 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 200 0 0 578
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 LOCAL\nectop 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 200 0 0 562
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 LOCAL\nectop 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 200 0 0 109
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 LOCAL\nectop 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 200 0 0 548
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 LOCAL\nectop 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 200 0 0 237
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 LOCAL\nectop 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 200 0 0 237
2016-08-29 09:08:36 192.168.50.100 POST /ews/exchange.asmx - 443 LOCAL\nectop 192.168.51.153 Mac+OS+X/10.11.6+(15G31);+ExchangeWebServices/6.0+(243); - 200 0 0 237