Why so many "page not found" for statistics.php?
If file exists, but Drupal still captures calls to it, it may mean one of few things:


Obviously this happens if filename is misspelled in one place (request or filesystem) but not the other (or misspelled differently in both).
As above, sometimes this happens when paths are (slightly) different.
Path you see in your address bar and path your server use to test for file existence are not the same thing. May look the same, but may not, depends on your .htaccess, nginx.conf or whatever you use to control that.
Language prefixes may break things. If file /sites/default/files/example.jpg exists, server may fail to find en/sites/default/files/example.jpg. If that's the case, and for any reason you can't fix broken code, you can try setup like this in your htaccess:

RewriteCond %{REQUEST_FILENAME} ^(en|de|fr)/(.*)$ [NC]
RewriteCond %2 -f
RewriteRule ^(en|de|fr)/(.*)$ $2 [NC,QSA,L]


Notice, I didn't test it. OP preferred to use %{DOCUMENT_ROOT}%{REQUEST_URI} instead of %{REQUEST_FILENAME} to deal with the potential dependencies on the environment/timing etc - that are implementation details beyond the scope of this site, I believe. The logic behind it is, line by line:


Test if request filename contains one of the specified prefixes (should match your site's languages.
Test if file without prefix (second regex group) is there, and really is a file.
If so, rewrite request to remove language prefix.


Apply this logic and you should be good. Actual implementation may vary, but logic you want is just that.