ERROR: The templates below matched known security issues. Please review them[Help]

[Nathan Malcolm]

Replace:

{$monthnames[$prev_month['month']]}

with:

{$prev_month['month']}

and:

{$monthnames[$next_month['month']]}

with:

{$next_month['month']}

It's just the way MyBB detects potential security issues. Nothing to worry about.