thanks to JD for his great explanation and non-patronising answer.
plusalpha
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I had this happen ... for me it turned out that I was signed into the MAC using a different Apple ID.
In Xcode I was unable to sign in using either ID, and when I tried I got the "session expired" message.
I signed OUT of the MAC ... so no Apple ID signed into the Mac.
I was able to delete the Apple ID and sign in to XCODE
then I was able to sign in to the Mac again (using the different Apple ID)