The problem is 100% reproducable on differnet servers where sogo is installed.

In my opinion this should have a high priority because loosing appointments is a horrible thing for companies.

Hello, could you be extra specific on the steps to reproduce it?
What is your external server?
By 'Apply it on sogo', you mean accept the invitation from the mail received?
How and what exactly do the owner when moving all elements?

What is your external server?
The external server is a sogo server, but it will work with others too.

By 'Apply it on sogo', you mean accept the invitation from the mail received?
By 'Apply it on sogo' i mean to accept the appointment from the received mail.

How and what exactly do the owner when moving all elements?
Just to move the serial appointment in sogo and select all future events for this serial.

So i've just test it and all is good on my side. My test:
Account A gmail, Account B sogo.

A make a recurrent event each Thursday with B.
B receives the invitation by mail and accept.
B see all the events correctly

A modifies only one event to be on Wednesday
B receives another mail and accept.
B see all the events correctly and at the right day (one on Wednesday and all the others on Thurday).

A modifies another event which was Thursday, and choose to move it and all the following recurrences to Wednesday.
B receives a mail and accepts.
B see all the events correctly (All events on Wednesday except the ones prior to the recurrences modified by A)

Seems to be a difference regarding the sending server.

The problem appears for 100% if both servers are sogo.

Right now i tried it from apple mail fo sogo and there is a other problem. (Sogo can't accept it, you can press it but it does nothing)

Ok I reproduce the bug