Ich habe es mittlerweile hinbekommen indem ich im UltraEdit-Forum etwas geschmökert habe.
Das Entscheidende ist zusätzlich die beiden Zeilen
/Open Comment Fold Strings = ";FOLD"
/Close Comment Fold Strings = ";ENDFOLD"
zu verwenden. Dann klappt's.

Also der Kopf etwa so:
/L14"KRL - KUKA Robot Language" Nocase Line Comment = ; String Chars = "' File Extensions = SRC DAT KRL SUB KFD
/Delimiters = -~*()|{}[]<>="' ,.?:
/Indent Strings = "then" "loop" "repeat" "for" "switch" "while" "else"
/Unindent Strings = "endfor" "endif" "endwhile" "endloop" "end" "endfct" "endswitch" "else" "until"
/Open Fold Strings = ";FOLD"
/Close Fold Strings = ";ENDFOLD"
/Open Comment Fold Strings = ";FOLD"
/Close Comment Fold Strings = ";ENDFOLD"