It looks like Cider is released as a Flatpak or AppImage. I’m not super familiar with building these/troubleshooting, but the error messages you are getting look like a poorly packaged app (Failed to load module “xapp-gtk3-module”).

IMHO, you should consider doing more troubleshooting on Kmail. I’ve never used it personally, but from my understanding, it’s a stable program and shouldn’t have problems doing the basics of email, like you’re reporting.