Hi Federico,
> In a nutshell, Geary marks the thread as read correctly in the thread view (central pane), but it fails to sync with the server. I think.
>
> Did anybody experience the same?
> Should I open a new issue on the tracker?
I can confirm this behaviour for a single account, but I was not able to capture a trace yet. It seems to be sporadic for a single account with a single alias.
Best
Bernhard