For no apparent reason, the site has crashed showing:

"XOOPS path check: Script is not inside XOOPS_ROOT_PATH and cannot run."

No changes have been made on the server or in the XOOPS configuration files leading up this event, and my other XOOPS installations for the same account on the same server still work fine. The database and application files all seem fully intact as usual.

I have attemted, resulting in no success, changing:
define('XOOPS_CHECK_PATH', 1) to
define('XOOPS_CHECK_PATH', 0);

What could have caused this crash to occur and how does one rectify it?
Could something in the database have been corrupted?
What is the exact script in reference?

I am running 2.0.7.

Many thanks.