except when; excluding the case when
Fixed expression used to state an exception or condition, often in formal or written contexts. Literally 'removing the case when...'.
Except in an emergency, please do not use this door.
The game will be held as scheduled, except in the case of rain.
Compound of 場合 (case, situation) + を (object marker) + 除き (continuative form of 除く 'to exclude'). Literally 'excluding the case'.