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Before you call customer service, I'd check the door lock myself - often it's just laundry or dirt blocking the sensor, or the door isn't sitting properly in the seal. If that doesn't help and the door still won't unlock even after you turn it off, then the locking module is probably actually broken and needs to be replaced.
The E:21 error on Bosch means the door lock isn't working properly - either it's stuck or the sensor is reporting a problem. Before you call customer service, I'd first check if anything's blocking the door or if it's actually closing properly. Sometimes just turning it off and back on helps. If that doesn't work, the lock or electronic latch is probably shot - at that point you'll need a technician.
I had something similar last week - the machine threw the error in the middle of the cycle and I couldn't open the door. super frustrating when you're trying to wash diapers and everything gets stuck. but for me it was just nonsense: the seal on the door lock was completely clogged with detergent residue and lint, it got jammed. just wiped it clean and it worked again.
so first I'd check that - look around the door frame to see if anything's stuck in there or if the seal looks worn. then unplug the machine for a minute and turn it back on, sometimes the sensor resets itself. if the door still won't open, the locking module is probably shot and then you can't avoid calling the service people. it costs somewhere between 80-150 euros plus the trip, but with Bosch you're better off not trying to fix it yourself.
The others already explained the door lock mechanism, but there's one important thing that gets overlooked: if you need to open the machine because there's still something inside, Bosch usually has an emergency release lever at the bottom front of the unit. You can only reach it if you pry off the panel - it's absolutely crucial to know about this in case the door is really stuck and you can't wait.
For the practical checks: first actually look to see if maybe a piece of laundry is stuck between the door and the seal - this happens pretty often with socks and smaller items. The door should also close without any force. After that inspect the lock itself - sometimes the mechanism is just locked because the program crashed, and a complete restart already helps. Unplug it for a minute, plug it back in, start a new program.
If none of that works and the error keeps coming back, then unfortunately the sensor or the latch itself is done for. There's no way around calling customer service - because that part isn't just easy to swap out 😅
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