OK: Surprised by a problem with an existing site that's been under use for quite some time. On attempt to create a site (or pretty much anything), the site returned a blank page with:
A quick search turned up the answer here (thanks 'magge2'):
In short, the Web Application General Settings had 'Web Page Security Validation' turned off. Turning this On (in Central Administration) fixes the issue.
The weird part - it appears that no one actually changed this setting so what gives? After some head scratching, I cannot say for sure - either someone changed it (and either doesn't remember or want to admit it) or the latest Cumulative Update turned it off.
Either way, be forewarned!