When I access chwala directly, I get the error in chwala.log:
[15-Jan-2018 17:43:16,504048 +0100]: <0150umj4> PHP Error: Login failed. Unable to connect to ssl://mail.xxxx.xxxx:8182. Error: Connection refused (POST)
The 8182 looks like the problem and this is the forwarded port from the WAN side, which does not make any sense inside the LAN. In theory there should be a 443, or probably nothing.