Yes and no.
Yes because adding extra bracket in the else like :
resolve the problem.See horstl's post
But no because I would like to have a better explanation that what you gave me...
Actually,
Using brackets speeds up processing. If an alias uses too few brackets then the statement might be ambiguous and the alias will take longer to parse, might be parsed incorrectly, or might not be parsed at all
is probably an answer but then another question have to be answered, how mirc can parse a line, said the line is valid a first time, and then displaying such an error for the exact same line ?
It's not the first odd behavior with /while so I imagine this is normal, I just find the behavior really weird