minRequiredNonalphanumericCharacters را برابر 0 قرار دهید.
در WEB.CONFIG
<membership>
<providers>
<remove name="AspNetSqlMembershipProvider" />
<add name="AspNetSqlMembershipProvider"
type="System.Web.Security.SqlMembershipProvider, System.Web, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"
connectionStringName="LocalSqlServer"
enablePasswordRetrieval="false"
enablePasswordReset="true"
requiresQuestionAndAnswer="True"
applicationName="/"
requiresUniqueEmail="True"
passwordFormat="Hashed"
maxInvalidPasswordAttempts="5"
minRequiredPasswordLength="1"
minRequiredNonalphanumericCharacters="0"
passwordAttemptWindow="10"
passwordStrengthRegularExpression="" />
</providers>
</membership>
موفق باشید.