To confirm the issue, I installed ZEG-3.2.0 in VirtualBox on my Ubuntu 14.04 Desktop machine.

When I create a recurring task (daily) in Thunderbird, that has a reminder set (0 minutes before), I get an error when trying to complete it:

    An error occurred when writing to the calendar ZEG!
    Error code: MODIFICATION_FAILED
    Description: Status Code: 2147746065, The request cannot be processed.

    Server Replied with 502

(Also, a yellow warning triangle is shown in Thunderbird to the right of the calendar, and hoovering above it with the cursor shows "The calendar ZEG is momentarily not available. Synchronizing the calendar removes the warning.)

When I create a recurring task in Thunderbird, that has no reminder set, complete one of them, and views the tasks in the web interface, I get the Proxy error popup and the busy spinning round icon never stops spinning. If I then delete the recurrent task in Thunderbird, and updates the web interface, the busy icon disappears and there is no Proxy error popup.

When I create a recurring task in Thunderbird, that has no reminder set, complete one of them, and then try to set a reminder on the recurrent task, I get the same error as above.

I have also noted that it is not possible to create a recurrent task in the web interface.

I'm using Thunderbird 45.3.0 with Ligtning 4.7 and SOGo Connector 31.0.3 on Ubuntu 14.04 and I used the CalDAV URL (http://192.168.0.191/SOGo/dav/sogo1/Calendar/personal/) to access the calendar, giving it the name ZEG.