Error 400, but only sometimes and disappears via refresh

I recently started getting an Error message that reads:

“This site is currently not available. Please try again in a couple minutes or contact the administrator”.

The weird thing about that is, that the site has been running fine up until about a week ago and no changes have been made to the code on my part (Kirby is v2.0.0.)

Even stranger is, that the error always disappears after an immediate refresh and everything is back to normal.
It does not reappear for several minutes even when the site is being visited from different devices/locations.

Yet it always comes back in the end.

Has this happened to anyone else or does someone here have an idea what might be causing this?

I just ran the paneltest and there seems to be an error in line 184 of kirby.php.

The Message is

Call to a member function isErrorPage() on boolean
180        // visit the currently active page
181        $page = $site->visit($path);
183        // react on errors for invalid URLs
184        if($page->isErrorPage() and $page->uri() != $path) {
186          // get the filename
187          $filename = basename($path);
188          $pagepath = dirname($path);

After Updating to the newest version of Kirby, the same Error simply moved a few lines down.

As of now, I seem to have resolved the issue by adding the formerly removed Error-Folder back into the Content directory.
Just in case someone comes across the same issue.