I just copied/pasted the .htaccess file you posted and uploaded it to my son's WordPress blog and it behaved exactly as it should, no 500 server errors on accessing stats or normal blog pages.
I can only think that the file was somehow 'messed up' during editing or uploading. This is not uncommon due to differences in the way some editors on certain operating systems treat CR/LF characters at the end of lines.
You need to upload the .htaccess file in ASCII mode, or create/edit the file using one of the existing editors on your server. Another option would be to run the dos2unix command in the shell to fix any CR/LF problems.
Running the shell command below from within the domain web-directory might fix the issue.
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