Error in Export MSPDI (XML)

Hello,
I receive ad error when I try to export a Gantt in MSPDI.
The error seems on the tasks duration:

Jul 14, 2021 10:38:27 AM net.sourceforge.ganttproject.GPLogger log
INFO: [ProgressMonitorImpl] begin Task: name=Running export
Jul 14, 2021 10:38:27 AM net.sourceforge.ganttproject.GPLogger log
INFO: [ProgressMonitorImpl] begin subTask: name=Export project
Jul 14, 2021 10:38:27 AM net.sourceforge.ganttproject.GPLogger logToLogger
WARNING: **Export failure. Failed subtask: Export project**
java.lang.RuntimeException: Export failure. Failed subtask: Export project
	at net.sourceforge.ganttproject.export.ExporterBase$2.run(ExporterBase.java:167)
	at org.bardsoftware.impl.eclipsito.JobManagerImpl$GroupWorker.run(JobManagerImpl.java:163)
	at java.base/java.lang.Thread.run(Unknown Source)
**Caused by: java.lang.ClassCastException: class java.lang.String cannot be cast to class net.sf.mpxj.Duration** (java.lang.String is in module java.base of loader 'bootstrap'; net.sf.mpxj.Duration is in unnamed module of loader com.bardsoftware.eclipsito.runtime.BundleClassLoader @31cfe0f3)
	at net.sf.mpxj.common.FieldTypeHelper.valueIsNotDefault(FieldTypeHelper.java:364)
	at net.sf.mpxj.mspdi.MSPDIWriter.writeResourceExtendedAttributes(MSPDIWriter.java:1108)
	at net.sf.mpxj.mspdi.MSPDIWriter.writeResource(MSPDIWriter.java:1023)
	at net.sf.mpxj.mspdi.MSPDIWriter.writeResources(MSPDIWriter.java:927)
	at net.sf.mpxj.mspdi.MSPDIWriter.write(MSPDIWriter.java:224)
	at net.sf.mpxj.writer.AbstractProjectWriter.write(AbstractProjectWriter.java:55)
	at biz.ganttproject.impex.msproject2.ExporterToMsProjectFile$2.run(ExporterToMsProjectFile.java:120)
	at net.sourceforge.ganttproject.export.ExporterBase$2.run(ExporterBase.java:165)
	... 2 more

Please can you help me?
Regards,
Alessandro

Please share the project file.