lucy24 - 8:01 pm on Dec 21, 2012 (gmt 0)
Does your error page have a custom name and location or are you using the host's suggested name? (I stress: filename, not actual content.) I ran into that same 500-error problem when I switched from the default "/forbidden.html" to a group of files within a /boilerplate/ directory. It seems safe to assume that the top-level config file has a <Files> envelope covering those specific names, along with a generic ErrorDocument directive. If you use your own filename you have to add exemptions.
And if your boilerplate uses any includes, make sure those include files are also exempted. 8,000 guesses how I know this.