Using SimpleDateFormat in a rule

I have witten a rule, which ran along time without errors. Here are the crucial code lines:

var SimpleDateFormat dfo = new SimpleDateFormat( "dd.MM.yyyy HH:mm:ss" )
 val calr = Calendar::getInstance
 sendTelegram("bot1", "Alarm wurde deaktiviert um " + dfo.format(calr.time))

When starting the Openhab server, the openhab.log shows

Variable 'dfo' on rule file 'housealarm.rules' cannot be initialized with value 'org.eclipse.xtext.xbase.impl.XConstructorCallImplCustom@111b8d (invalidFeatureIssueCode: null, validFeature: false, explicitConstructorCall: true, anonymousClassConstructorCall: false)': An error occurred during the script execution: null

This strange behaviour happens since one of my last Openhab updates. Any hints for me?