NPE while retrieving Messages sent as base64 encoded plaintext

After retrieving the first message sent as

Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: base64

the following log-entry occurs every minute in the log-file:

WARN [atlassian.confluence.mail.DefaultMailAccountManager] Could not store message within Confluence: [com.sun.mail.pop3.POP3Message@45f03d] - this message will be left on the server
com.atlassian.confluence.core.ConfluenceException: Could not store incoming mail: null
at com.atlassian.confluence.mail.DefaultMailContentManager.storeIncomingMail(DefaultMailContentManager.java:222)
at sun.reflect.GeneratedMethodAccessor57.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:324)
at org.springframework.aop.framework.AopProxyUtils.invokeJoinpointUsingReflection(AopProxyUtils.java:61)
at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:149)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:116)
at com.atlassian.confluence.util.profiling.SpringProfilingInterceptor.invoke(SpringProfilingInterceptor.java:16)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:138)
at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:56)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:138)
at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:152)
at $Proxy13.storeIncomingMail(Unknown Source)
at com.atlassian.confluence.mail.DefaultMailAccountManager.storeMessage(DefaultMailAccountManager.java:139)
at com.atlassian.confluence.mail.DefaultMailAccountManager.poll(DefaultMailAccountManager.java:90)
at com.atlassian.confluence.mail.DefaultMailAccountManager.poll(DefaultMailAccountManager.java:48)
at com.atlassian.confluence.mail.DefaultMailAccountManager.pollAllSpaces(DefaultMailAccountManager.java:274)
at sun.reflect.GeneratedMethodAccessor133.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:324)
at org.springframework.aop.framework.AopProxyUtils.invokeJoinpointUsingReflection(AopProxyUtils.java:61)
at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:149)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:116)
at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:56)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:138)
at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:152)
at $Proxy20.pollAllSpaces(Unknown Source)
at com.atlassian.confluence.mail.jobs.MailPollJob.executeInternal(MailPollJob.java:19)
at org.springframework.scheduling.quartz.QuartzJobBean.execute(QuartzJobBean.java:66)
at org.quartz.core.JobRunShell.run(JobRunShell.java:191)
at org.quartz.simpl.SimpleThreadPool$WorkerThread.run(SimpleThreadPool.java:516)
Caused by: java.lang.NullPointerException
at com.sun.mail.handlers.text_plain.writeTo(text_plain.java:105)
at javax.activation.ObjectDataContentHandler.writeTo(DataHandler.java:839)
at javax.activation.DataHandler.writeTo(DataHandler.java:295)
at javax.mail.internet.MimeUtility.getEncoding(MimeUtility.java:230)
at javax.mail.internet.MimeBodyPart.updateHeaders(MimeBodyPart.java:1065)
at javax.mail.internet.MimeMessage.updateHeaders(MimeMessage.java:1923)
at javax.mail.internet.MimeMessage.saveChanges(MimeMessage.java:1904)
at javax.mail.internet.MimeMessage.writeTo(MimeMessage.java:1604)
at javax.mail.internet.MimeMessage.writeTo(MimeMessage.java:1583)
at com.atlassian.confluence.mail.Mail.setMimeMessage(Mail.java:220)
at com.atlassian.confluence.mail.DefaultMailContentManager.storeIncomingMail(DefaultMailContentManager.java:210)
... 30 more