I'm updating from Merlin's 386.5 to 386.5.2 on my AC3100, hardwired AP mode to my AC86U. I have never enabled AIMesh.Welcome to the forums @slieveen.
What firmware do you have now on your RT-AC3100? Do you have any USB devices plugged into it?
Have you extracted the download and verified the hash on the file you're using to upgrade with? Are you double-checking you're using that same file?
With all the above checks (above) passing, and assuming you're using a USB device too, 'Safely' remove all USB drives from the GUI, and then physically remove them from the router too. Reboot the router and wait for it to settle after it has come up again (around 5 minutes). Now try reflashing it again.
Note that you may need to attempt to flash it a few times, just keep trying (you won't need to redo all the steps above if they were done properly already).
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