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CTRL + H, Search & Replace sometimes works, sometimes not

Good day! I routinely use Ctrl/H to initiate Search and replace operations. Sometimes it works in Studio 2015 (on a Windows 10 platform) and sometimes it doesn't, I get a nasty popup window: "Error  InvalidArgument=Value of '1' is not valid for 'index'.  Parameter name: index"  Why is Ctrl H sometimes working and sometimes not? If I close/reopend Studio, it generally works again. Has this happened to anyone? Can Paul perhaps explain and provide a quick solution? Thanks, have a great day!