Cookie/Session problem for some users, not all

I have found a lot posts about the 'Session expired or cookies are not enabled in your browser' problem. However they all seemed to deal with the situation where all users experience this.

I have a situation in which most users are not bothered with this, but some do. There are a bit too many people reporting this per day to blame it on cookie settings on their website.

I have around 30 to 50 new users daily, somehow about 10% reports this problem.

I have turned of caching in Joomla, and I have instructed these users to check their cookiesettings and clear their cache. This did not solve the problem in any of these cases.

Where should I search next?

Probably using privacy browser extensions like ghostry and blocking all cookies, waiting too long to submit the form causing their session to expire and thus the cookie as well (e.g. navigating to registration and walking away for awhile to come back to it later), or maybe an odd or out of date browser. Without a reliable way to confirm the issue there's not much more I can suggest beyond maybe try increasing session lifetime in Joomla global configuration to see if that helps and be sure not to cache any page with a HTML form on it (typically means you can't use Joomla page caching plugin).

I have done some more investigation - session time already is 600 seconds. That should not be too little should it?

I may have narrowed the problem down to Mac Users. As far as I can tell, all reports are from users on Macs, with different browsers. But I must admit, this is just about 10 users that I was able to get this information on.

Problem occured on browsers with no plugins, even on fresh installed browsers...

...today a few new cases, al Mac users. And I can confirm it is not on all Macs. I have a OSX10.11.6 (El Captain) that has no trouble...

What browsers were the users using? Have they tried different browsers? Where exactly are they seeing this? Just registration? Login? Profile edit? etc..

The only reports are from the registration page.

And most of them have tried different browsers. Safara, Chrome and Firefox all have this problem...

The versions of MacOS are all different... and I have confirmed that it is not happening to 100% of the users. As far as the users can tell, their browsers have no plugins.

I must say that I did not get any reports since about a week... but traffic has been slower than it was a week ago.