Unfortunately it seems that now when Clearspace (I'm using Clearspace 2.5.2 -- I failed to mention that earlier, sorry) attempts to load the plugin, the classloader can't find the UserProvider class. Here is the relavant output from the logs:

16:33:19,973 [main] ERROR com.jivesoftware.community.lifecycle.JiveApplication - Error loading class [com.appcelerator.clearspace.AppceleratorUserProvider] for bean with name 'appceleratorUserProvider' defined in URL [file:/Users/martin/Downloads/jive_clearspace_community_standalone_2_5_2/jiveHome/cache/plugins/appceleratorauth/spring.xml]: problem with class file or dependent class; nested exception is java.lang.NoClassDefFoundError: com/jivesoftware/spi/user/UserProvider
org.springframework.beans.factory.CannotLoadBeanClassException: Error loading class [com.appcelerator.clearspace.AppceleratorUserProvider] for bean with name 'appceleratorUserProvider' defined in URL [file:/Users/martin/Downloads/jive_clearspace_community_standalone_2_5_2/jiveHome/cache/plugins/appceleratorauth/spring.xml]: problem with class file or dependent class; nested exception is java.lang.NoClassDefFoundError: com/jivesoftware/spi/user/UserProvider

Here is my spring.xml file where I've defined both the UserProvider and an AuthFilter I've been working on:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:aop="http://www.springframework.org/schema/aop"
       xmlns:util="http://www.springframework.org/schema/util"
       xmlns:tx="http://www.springframework.org/schema/tx"
       xmlns:dwr="http://www.directwebremoting.org/schema/spring-dwr"
       xmlns:cxf="http://cxf.apache.org/core"
       xmlns:jaxws="http://cxf.apache.org/jaxws"
       xsi:schemaLocation="
http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-2.0.xsd
http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.0.xsd
http://www.directwebremoting.org/schema/spring-dwr http://www.directwebremoting.org/schema/spring-dwr-2.0.xsd
http://cxf.apache.org/jaxws http://cxf.apache.org/schemas/jaxws.xsd
http://cxf.apache.org/core http://cxf.apache.org/schemas/core.xsd"
       default-autowire="no">

    <bean id="filterChainProxy" class="org.acegisecurity.util.FilterChainProxy">
        <property name="filterInvocationDefinitionSource">
            <value>
                CONVERT_URL_TO_LOWERCASE_BEFORE_COMPARISON
                PATTERN_TYPE_APACHE_ANT
                /upgrade/**=httpSessionContextIntegrationFilter, upgradeAuthenticationFilter, upgradeExceptionTranslationFilter,jiveAuthenticationTranslationFilter
                /post-upgrade/**=httpSessionContextIntegrationFilter, postUpgradeAuthenticationFilter, postUpgradeExceptionTranslationFilter,jiveAuthenticationTranslationFilter
                /admin/**=httpSessionContextIntegrationFilter, sessionTrackingFilter, adminAuthenticationFilter, openfireAuthenticationFilter, adminExceptionTranslationFilter,jiveAuthenticationTranslationFilter
                /rpc/xmlrpc=wsRequireSSLFilter, httpSessionContextIntegrationFilter, basicAuthenticationFilter, wsExceptionTranslator, jiveAuthenticationTranslationFilter, wsAccessTypeCheckFilter
                /rpc/rest/**=wsRequireSSLFilter, httpSessionContextIntegrationFilter, basicAuthenticationFilter, wsExceptionTranslator, jiveAuthenticationTranslationFilter, wsAccessTypeCheckFilter
                /rpc/soap/**=wsRequireSSLFilter, httpSessionContextIntegrationFilter, jiveAuthenticationTranslationFilter
                /**=httpSessionContextIntegrationFilter, sessionTrackingFilter, appceleratorAuthFilter, formAuthenticationFilter, rememberMeProcessingFilter, feedBasicAuthenticationFilter,exceptionTranslationFilter,jiveAuthenticationTranslationFilter
            </value>
        </property>
    </bean>
   
    <!--  Define a new filter for externally-managed users. Inject dependencies as needed. -->
    <bean id="appceleratorAuthFilter" class="com.appcelerator.clearspace.AppceleratorAuthFilter">
        <property name="userManager" ref="userManagerImpl"/>
    </bean>
   
    <bean id="userManagerImpl" class="com.jivesoftware.base.profile.MultiProviderUserManager">
        <property name="userIDCache" ref="userIDCache"/>
        <property name="userCache" ref="userCache"/>
        <property name="userDAO" ref="userDAO"/>
        <property name="userContentExistsDAO" ref="userContentExistsDAO"/>
        <property name="saltSource" ref="saltSource"/>
        <property name="passwordEncoder" ref="passwordEncoder"/>
        <property name="eventDispatcher" ref="userEventDispatcher"/>
        <property name="userContainerManager" ref="userContainerManagerImpl"/>
        <property name="userProviders">
            <list>
                <ref bean="appceleratorUserProvider"/>
            </list>
        </property>
    </bean>

    <bean id="appceleratorUserProvider" class="com.appcelerator.clearspace.AppceleratorUserProvider">
</bean>

</beans>

I'm not sure what's going on here, but perhaps I have a configuration issue?

Yep, the import is there.

I actually did base my plugin on the custom-authentication plugin.

Could this be related to my use of the evaluation license? Our organization has a license, but for the purposes of plugin development, I'm using the evaluation license.

Also, I do notice that above the exception that I included I see:

Exception in thread "pool-1-thread-1" java.util.concurrent.RejectedExecutionException
    at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:1477)
    at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:384)
    at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:856)
    at java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:31)
    at com.jivesoftware.base.event.v2.NonBlockingEventDispatcherImpl$Watcher.run(NonBlockingEventDispatcherImpl.java:159)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:650)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:675)
    at java.lang.Thread.run(Thread.java:613)

Nope, the exception is only thrown when the plugin is loaded in Clearspace.