Fix for "Your session has expired. Please log in." and potentially other login issues:
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Close iTunes if it's opened. Kill it from the Task Manager.
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Open Start > Run > type in %ProgramData% and hit enter.
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Next, navigate inside Apple Computer > iTunes. The complete path will be: C:\ProgramData\Apple Computer\iTunes
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Now you can safely rename the "adi" folder to "adi.bak" or just delete it.
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Open Sideloadly and then try sideloading an IPA and you will be asked for your Apple ID password again.