So did you just complete the red-starred fields? When I did so for my latest online report a couple of weeks ago, this, too, was initially rejected on the grounds of "Incomplete information provided". So I completed all remaining fields and resubmitted, with approval by my office (Rayong) being forthcoming the next day.
I strongly suspect that the sole piece of additional info which is considered so absolutely desperately essentially vital by IMM as a prerequisite to their approval of online 90-day reports is your phone number. For some mysterious reason this is an optional field in the online system, yet mandatory on a paper TM47! However, I was taking no chances, so gave 'em the full Monty instead.