My b2evolution Version: 1.10.x

I'm still running 1.10.2, and out of the blue today this message appears at the top of my blog:

" Warning: $ReqPath could not be set. Probably an odd IIS problem.

Go to your phpinfo page, look for occurences of /install/phpinfo.php and copy all lines containing this to the forum. Also specify what webserver you're running on.
(If you have deleted your install folder – what is recommended after successful setup – you have to upload it again before doing this). "

What does this mean, and what could have caused it?