I can’t reproduce it. I doubleclick in my calender, fill out the form, hit Save and it saves and closes. (latest WebApp, can’t easily detect the version number)
Can you reproduce the problem with
DeskApp (is it a client or server problem?)
another mail account (store problem?)
another browser (Browser incompatibily, Plug-In problem?)
another computer (any local problem?)
with a mobile browser (bypass any web proxy)
These questions were those I’d ask myself to track down the issue.
And: did you check the log apache log and errorlog files, just to be sure?