There was an error how URIs were handled in our custom XSL loader
that rewrites include URIs to point to admin dir if the include
is not found in the current dir.

The check for the file would not work if the path contains chars
that are usually escaped in URIs, like a space (%20), as we get
a already encoded version of that URI in the loader and did not
decode it before checking the existence of the file.

(Fix #2249)