LS Retail PluscFront license screen comes pop up

We are using Navision 5.0 with SQL Server 2008 database.For data replication from stores we use data replicator scheduler.

From the past few months we are facing a problem with the scheduler, Every few hours a popup screen of PluscFront license keeps coming which stops the scheduler job.We have to click OK button and then only the scheduler will continue.The license key and other details are all showing correctly in the popup screen.Anyone faced the same problem...Please help...