I tested a similar scenario and everything on my side is working as expected. I do not have the implementation of your PleaseWaitForm and I cannot further investigate it.
I noticed that you were reading some data from Excel. Perhaps it might be a good idea this process to be wrapped up in a BackgroundWorker, an example is discussed here. Additional information and other examples about the RadWaitingBar element you can find in this section of our documentation.
I hope this helps. Should you have further questions please do not hesitate to write back.
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