Sometimes figuring out the source of an error is as easy as doing a Google search on the message being shown instead of what should be shown. In other situations that isn’t the case. We recently had someone contact us for Joomla support that had a website where the website’s menu had disappeared and for most pages of the website they were getting a message that “You are not authorised to view this resource.” instead of the page’s content.
If you were to Google that message you get a lot of results with a lot of different possible resolutions. One of the first results is for a slightly different message (with an additional sentence after that one) and there were eighteen possible causes listed for the issue in that result.
In this situation, looking at what was shown in the PHP error log (and was shown when the display of errors was turned on) made it look like the problem could have been caused by the PHP version being used. The version of PHP in use was version 7, which considering that the Joomla and the installed extensions where several years out of date, could cause errors to occur. It would appear that web host had automatically changed to that version and lowering the version back down to PHP 5.6 got the website working again.
PHP 5.6 is supported with security updates until the end of next year, so moving to a newer version isn’t necessary yet.
While extensions may still have issues with the newer version of PHP, Joomla introduced official support for PHP 7 with Joomla 3.5, which was released in March of last year. Considering that numerous security fixes have been released since that version, you should already have upgraded Joomla to a newer version than that.