Hi JBlrjck,
We see you're running into an authentication error when using Migration Assistant on your new Mac. We can certainly understand the need to get your data transferred across, and we'd like to assist.
For this particular issue, try resetting the SMC of the Mac. The steps for that can be found here: How to reset the SMC of your Mac
If this doesn't work the first time, you may need to try it a second time, just to be sure.
After that, try using Migration Assistant again as normal.
Take care!