Hi Simon Hudson,
Since we can't reproduce your situation, we need to narrow down the problem and try some things.
1, Please check if the user's permission level for the file to be checked out is Contribute.
2, If the user's permission level is fine but the problem persists, try removing the user from the group and reassigning the Contribute permission level. Once done, please wait a few minutes, there will be a slight delay for this configuration to take effect. Clear the browser data and then log back in to see if the problem still exists.
3, Please try to add the user to other site's group with Contribute permission level, whether the user can check out files of other sites.
4, Try to use a different browser or your browser's InPrivate mode to see if the same error occurs.
5, Please try logging into the user's account from another computer to see if the problem still exists.
6, Please check if the user's permission level for Document Library is Contribute.
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