This appears to apply to all (my?) tools. It seems to be new since the eqiad migration.
Example:
http://tools.wmflabs.org/widar/index.php - works
http://tools.wmflabs.org/widar/ - works
http://tools.wmflabs.org/widar - "Internal error" (note the missing '/')
Tried different lighttpd configs, to no avail.
Version: unspecified
Severity: normal