Thanks Phaedrux. Mechanically, nothing has changed on these printers and mine are workhorses. The error message seems logical and your first post above made sense so I tried -15mm drop in Z on my machine but still getting this error.
It is true that a movement in XY on a delta causes at least 1 carriage to move up the tower and that does seem to be what this error message is telling us. But at 15mm down from the home switches there should be enough room for small X-Y movements. So as a test I did the final downward move to:
G1 H2 X-100 Y-100 Z-100 F6000 ; move down a few mm so that the nozzle can be centred
So now I see the home sequence, the slow descent of 100mm, then a final X-Y adjustment presumably for the M666 endstop offset adjustments and THEN the error is raised.
David, you asked about my M665 and M666 before, they are copied from my 2.x config.g. They seem correct. Hmm, this got me thinking, maybe my config-override.g has some bad stuff in it. I commented out the M665 and M666 in it and now homing seems to be working again. Here is what I commented out in config-override:
;M665 L376.890:376.890:376.890 R209.670 H450.462 B140.0 X0.114 Y0.283 Z0.000
;M666 X0.107 Y-0.648 Z0.542 A0.00 B0.00