[Dardania]

Maybe the Mac is the straw that breaks the camels back. You might have other leakages elsewhere (water heating is always a good one) that just fall below the threshold…and the Mac tips it over the GFCI? Way to check would be to disconnect / isolate earthing downstream of GFCI and try with Mac then.. also to measure the GFCI leakage current (electricians usually have the instrumentation to do so)

[sdflhasjd]

The fault only happens the instant the connector mates with the back of the mac. If I connect the cable into back first then plug it into the wall socket, there's no issue.