This was really great advice! Thank you!!
I hex-edited misc and got into recovery. There I selected fastboot and set the active slot to b. Now everything is working again.
Btw. I also tried manually sideloading the decrypted update.zip, but it was incompatible, because device-type "NovaAirC" was excepted, but it got "BOOX". But I guess I don't need to bother if it is working now...