I receive an invitation to an event via email.
I use thunderbird and accept the invitation.
I use tine20 for my calendar.
I can see the event in my calendar.
But the sender does not see that I accepted the event.
I have no clue how to debug this.
What could be wrong?
I use thunderbird 52.8.0
AFAIK the sender uses outlook for his calendar.
If you have multiple email addresses setup in Thunderbird you could be experiencing a problem where the email to accept the invite is being sent using a different address, thus they aren't seeing the accept in their calendar.
https://support.mozilla.org/en-US/questions/1206863
You may also want to try running Thunderbird 60 (newest version) as some of these problems are considered bugs - it's hard to tell if they have been fixed since there is a large chain of duplicates in their bug tracker. (source)
Download for the latest version of Thunderbird is on their homepage - extracting and running that version should load the same settings as your existing installation. For safety you can package up your current settings in
~/.thunderbird
before using it.