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I'd just like to make a quick suggestion to add a 'Migration' page to the documentation and a more useful error message which covers the unexpected behaviour you get when migrating the forum (or, in my case, simply renaming the containing directory). It produces this initially cryptic message:
Fatal error: Class 'Gdn' not found in [Path redacted]/bootstrap.php on line 47
After spending a while reading and re-reading the bootstrap and not understanding how the autoloader could possibly be failing because of a change in directory name when all paths are set dynamically at application startup, I finally stumbled upon the cache dir and the cached ini's within.
Perhaps a more verbose error message, or the autoloader could refresh the cache clearing if the directories don't appear to be valid?