Invalid Token when logging in

Every so often when students log in to the website, they get a white page with "Invalid Token" on it. If they push back, then they are logged in, but only to Joomla - if they try to go to a Moodle page, they get given the Moodle login form.

What's frustrating is that I cannot reliably replicate it, yet it happens fairly often - in a class of 30 students, I'd say 3-4 will get it each time.

It doesn't seem to always be the same users either.

Do you have any ideas about what could be causing this? Is there any logging anywhere I can see/turn on (on Moodle, Joomla or Joomdle) to find out what's going wrong?

The system check is all green and it seems to work a lot of the time, so I am stumped!

It's been happening since the start pretty much, I was hoping there might be something in 1.0 that would stop it, but unfortunately not.

Thank you!

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Joomla: 3.3.6
Moodle: 2.8.1
Joomdle: 1.0

This is normally a session management / caching issue and can be seen in Joomla in general. It does seem to present itself more often when using Joomdle because of the need to manage login sessions in both Joomla and Moodle.

What I do to prevent such messages is:
- ensure I have no caching for my Joomla login modules
- make my sessions life time long "enough" eg. rather than 5 or 15 minutes, I set my session life time at a few hours
- install a invalid token redirection. There are a few plugins for Joomla specifically designed for this Invalid Token message. I use it to intercept the message (nothing worse than a white page with a silly error message) and redirect to an error page saying something like your session has been logged out due to inactivity or whatever and ask the user to log back in. You could even include a button (ie push to continue) where the button is a immediate logout plugin to ensure no session is active.

In my observations, this message has mostly been due to cache on login modules. You can google joomla invalid token and will see other suggestions like expired cookies and disabling browser cache in joomla system config but I have not seen it caused by these as much.

Ah thank you for the help Chris, especially as it doesn't seem to be a Joomdle issue! Very much appreciated, going to try some of those things now.