I don’t see the problem. DAVdroid will encode “@” to %40 and Radicale is supported for a long time. As this bug report is quite old and has already been resolved, can you please create a new issue, including detailled steps to reproduce, logs etc?
unable to sync with owncloud calendar
Davdroid is unable to contact with owncloud calendar.
0_1451112603719_davdroid-debug980229518.txt Check uploaded file for debug info.
As you can see in the logs, the TCP connections have been refused. This is a server or firewall problem and very likely not a problem of DAVdroid. Please contact your administrator, or if you are the administrator, fix the network problems.