The same Problem exits, when accepting an invitation with an ActiveSync client.
The problem seems to be a wrong formated DTSTART and DTEND field.
This is no SOGo (only) problem.

http://stackoverflow.com/questions/37622750/getting-not-supported-calendar-message-ics-attachment-with-outlook-email-invit

I think it's an Exchange issue, but should be handled by SOGo, as Microsoft wouldn't change it's behavior.

When testing with Outlook 2016 NOT connected with SOGo in any way (so just over a standard IMAP server), all works fine.

The IMIP reply sent by SOGo to Outlook is correctly processed.

The problem isn't by Outlook, it's an Exchange problem.

Even with 3.2.5 the problem exists: User A is on Exchange (e.g. Office 365), no matter if he uses Outlook or Exchange WebMail. User A sends an invitation to User B, User B is on SOGo. User B accepts invitation, User A gets the response with the "not-supported-message.ics attachment.

The problem could be caused by a valarm element in the IMIP reply.
Can you try to send an invite (user A on Exchange) without a reminder and check whether the is behavior different?

I can confirm what tfu wrote: without a reminder all works fine, there is no "not-supported.message" ics attachment and Outlook show the response in its calendar. So it seems the problem is caused by a VALARM entry.

If I read section 3.2.3 https://tools.ietf.org/html/rfc2446#section-3.2.3 correctly valarm should/must not be percent in a calendar reply.

Fix pushed - please test.

It works. Thank you.

But during my last test, I found another issue.
I created an invitation from an Exchange user to an SOGo user => it works
The SOGo user accepts the invitation => (wow) it works
As it was a test invitation, the Exchange user send a cancellation to the SOGo user => The date in the SOGo calendar still exists :(

Should I open a new issue for this behavior?

For me it works as well, thanks a lot!

I cannot confirm the problem with the cancellation described above. My SOGo user receives an email upon cancellation (you have to instruct Outlook or Exchange WebMail to send this notification) and I see a button "remove from calendar". After clicking this button the appointment disappears.

My SOGo user also receives the cancellation mail.
But on my Activesync device, the date still exists.
Maybe this is a problem with Activesync (sorry that I forgot to give this information)

"The date in the SOGo calendar still exists"

That is normal - Exchange is an external system, and won't automatically delete the event on SOGo's calendar.

The user has to click on the "delete button" to wipe it.