Well, I definitely can't tell if the include path is correct or not if you're modifying the error, so... I'm of no use. Without knowing more, and assuming it's nothing to do with any files that have been modified, I would say this could have something to do with the .htaccess file being ignored by Apache. Perhaps it's also just a permissions issue with the .htaccess file, so you could make sure it's set to 644, perhaps 666. Inevitably, though, if the main site is working but the AdminCP is not, then we might need more information.
What hosting did you settle on, by the way? That information is also relevant, in case it has to do with their Apache settings and if they're respecting the .htaccess file properly.
You might have to ask
Hall of Famer directly about this one, in the end. I also ended up with a host where I couldn't access the AdminCP.
Inevitably, if you just want to develop a site for the fun of it before paying for hosting, I'll always recommend you just
host it yourself by turning your computer into a mini web server, rather than trying to use 'free' online hosting.