I tested your bot, and "hello" responded with "hello" (from your * pattern).
I tried several other patterns from your script, and could not recreate any errors.
If you have a sequence of phrases that cause to error, please included them so I can recreate the issue.
If this error causes further chats to give the same error, my guess would be the bot's cache is some how corrupted. You can try clicking on the "Clear Cache" button from the Knowledge page to reset its cache.