The "garbage" characters in the "Password to access exam (optional)" 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:
Steps to fix the access password
Start with an exam that does not required 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.