Hi Paolo,
Can you have a look in the owncloud.log and the apache error.log file and paste the error messages regarding the expired token here.
As your ownCloud version is a little older I’ve had a quick look on Github whether I could find related bugs, but was unsuccessful for now (doesn’t mean there isn’t one, just that I didn’t find one). Perhaps you could also have a look: https://github.com/owncloud/core/issues/
I think additionally you could have a look into the oc_authtoken which might be the corresponding database table for the generated tokens, after you’ve created a password reset email in the web interface.
I’m also not sure whether the ownCloud version you’re using actually still supports your PHP version, as 5.6 is quite old now as well…
Cheers,
Erik