This is just a solution I stumbled on when encountering the following message after upgrading firmware on a iR2535i: "After inserting the toner and drum press the Start key to start mixing the toner."

When the procedure failed (which it did every time I followed it), the following message was displayed: "An error occurred when mixing the toner. Check the toner and drum, then turn the main power OFF and ON."

Power-cycling the copier did not resolve the issue either.

What finally worked is going in to Service Mode > #CLEAR > TONER-INSTALLED > ON

That finally brought it to ready mode.