c) there's a simple workaround (as you have now implemented).
chewitt, unfortunately i've to realize that my connman-proxy config doesn't solve the problem.
As soon as i've reset "EnableOnlineCheck=true", i've noticed that connman is doing his online checks anyway without using the proxy configuration in the settings file. 
Thanks chewitt, so i'm thinking it's more a Feature Request than a Bug. 
While writing this, i've tried to get connmann using my Proxy Configuration manually. My Libreelec-Raspi is connected over ethernet, so my steps for the connman-configuration are:
connmanctl services
*AR Wired ethernet_b827eb498c13_cable
connmanctl config ethernet_b827eb498c13_cable --proxy manual http://username:password@proxy-ip:portThis configuration will be appended to "/storage/.cache/connman/ethernet_b827eb498c13_cable/settings" and will survive a reboot.
The settings file looks like:
IPv4.method=manual
IPv4.netmask_prefixlen=24
IPv4.local_address=192.168.x.y
IPv4.gateway=192.168.x.z
IPv6.method=off
IPv6.privacy=disabled
Nameservers=192.168.y.z;
Timeservers=0.pool.ntp.org;1.pool.ntp.org;2.pool.ntp.org;
Domains=my.domain;
Proxy.Method=manual
Proxy.Servers=http://username:password@proxy-ip:port;I think the ip Settings in this file like address,gateway,dns .. had been read out of the kodi configs before and then given to connman.
Why shouldn't it be possible for Libreelec to do the same for the proxy settings?
I've a Raspi with Libreelec 12.0.0 running in a ipv4-LAN behind a http-proxy.
The Proxy is the only way to connect to the internet.
All other traffic will be logged and blocked by the firewall.
In Kodi I've filled out the Proxy-Settings, and Kodi himself is using the Proxy correctly.
But after starting Libreelec, the firewall-logs are continuously showing blocked IP's initiated by Libreelec:
82.165.8.211:80
212.227.81.55:80
Obviously they are caused by the Connman Service during his online check procedure.
Connman is doing these online check request for "ipv4.connman.net" without using the proxy configuration of Kodi and of course the checks will fail.
Ok, when disabling these checks in the connman conf file with "EnableOnlineCheck=false" these checks are breaking off.
But, why isn't Libreelec/connman using the Proxy-config from Kodi for these checks?
So far as i Know, this is the only place in Libreelec where a proxy can be defined.