I found a possible workaround by modifying the following section in applicationContext-externalAuth-LDAP.xml:
<bean id="ldapAuthenticationManager" class="com.jaspersoft.jasperserver.api.security.externalAuth.wrappers.spring.JSProviderManager">
<constructor-arg index="0">
<list>
<ref bean="ldapAuthenticationProvider"/>
<!--<ref bean="${bean.daoAuthenticationProvider}"/>-->
<!--anonymousAuthenticationProvider only needed if filterInvocationInterceptor.alwaysReauthenticate is set to true
<ref bean="anonymousAuthenticationProvider"/>-->
</list>
</constructor-arg>
</bean>
I commented out the usage of daoAuthenticationProvider. This solves the issue with the error 500 message, but it also means that the login for the internal jasperadmin account is disabled.