So. For whatever reason (too busy at the time; fog of war; sheer laziness) I didn’t bother looking in great detail at my globalvariables.groovy file when I sent it along to @devalex88 and the team to investigate. I certainly should have. Kicking myself now…
But let’s not get ahead of the story… here’s how it all went down, this morning…
I thought I’d give the KS 5.11 beta another try. Same issue as before. And the annoying thing, the Problems pane shows (showed) nothing.
Ah, but… it DID have something in it, but Katalon shows it as hidden on first heading in there. Click the header and all was revealed. Problem in GlobalVariables (surprise, surprise).
I double-clicked the entry and up popped the GV groovy file automagically produced by KS at startup of a test. It highlighted one particular variable:
"$WE_ACTIVE"
You’re ahead of me, now, right? 
In 5.4x (at least) that variable says:
'$WE_ACTIVE'
So, new mantra (to be adopted by the Katalon Devs, hopefully)
“Thou shalt not use double-quotes unless you can predict (or otherwise control) the content therein”
I sincerely hope this is the cause of many other failed upgrades.
UPDATE: