"Show Report" seemed unresponsive, until eventually it showed me a file on the finder called "eficheck.dump" which I was dumb enough to open and it's just a bunch of, to me, unintelligibly code (and some info about this mac)
That file is here
The folder you can't see there is called 'tmp' I think
Then I clicked 'don't send' and that closed that window.
eficheck is a normal automatic diagnostic that has found a discrepancy perhaps related to the firmware being outdated. This was possibly because the new OS wants to see a newer firmware version. That would be the best case because updating ought to fix it.
Worse case is that there's something either failing in the firmware or the new OS has some unauthorised code in it put there by bad people or some other software was recently installed that eficheck doesn't approve of.