More tests might help pin down the bug. Here is the complete config for completions.
[AutoComplete]
enable=True
popupwait=2000
[AutoComplete_cfgBindings]
force-open-completions=<Control-Key-space>
[AutoComplete_bindings]
autocomplete=<Key-Tab>
try-open-completions=<KeyRelease-period> <KeyRelease-slash> <KeyRelease-backslash>

Notes: config-extensions.def is initially opened and generically processed in various methods of configHandler.IdleConf, and on request, the config-extentions section of EditorWindow.py. cfgBindings can be (sensibly) reconfigured, the others not. Names that follow, before '=', become pseudoevents handles by methods in the

Alessandro, you said <Control-Key-space> works, <KeyRelease-period> does not.  How about <Key-Tab> and <KeyRelease-(back)slash>. The latter pair work within path strings when the preceding chars begin a path that exist on your system.  For instance, on Win 7, './ lists options in the current directory.

I have no idea why KeyRelease instead of Key is used above and below.  There is no key equivalent of releasing a mouse button in a different location than where pressed. Kevin, could there be a bug with KeyRelease on Mac?  Alessandro, you could test by making a backup of configextensions.def and removing 'Release' for 'period'.

The calltip files are
CallTips.py
CallTipWindow.py
and in config-extensions.def,

[CallTips]
enable=True
[CallTips_cfgBindings]
force-open-calltip=<Control-Key-backslash>
[CallTips_bindings]
try-open-calltip=<KeyRelease-parenleft>
refresh-calltip=<KeyRelease-parenright> <KeyRelease-0>

Alessandro, you said <KeyRelease-parenleft> does not work.  How abut <Control-Key-backslash> (after '(') to open?  Does <KeyRelease-parenright> close properly?

I do not know what <KeyRelease-0> (zero) is about.  It does not work for me.

For both boxes, clicking on the box should also close. Does it?
What about Edit -> Show Completions and Edit -> Show calltips for opening?