The "garbage" characters in the "Password to access exam (optional)" for Lockdown Browser (LDB) enabled exams can occur from other test settings or from a course copy or restore of a much older course. To resolve this, please do the following:
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Start with an exam that does not required require LDB. If the exam does require LDB, remove the optional password (as set via the LDB Dashboard) and then remove the LDB requirement entirely and save the settings.
Open the LDB Dashboard, and enable LDB as a requirement, then specify the optional password in the LDB Dashboard settings.