Sending mails with mu4e through protonmail-account gives error-message

Question

I've configured Emacs for my gmx and protonmail accounts using mu4easy. I can receive mails through both accounts and send mails through the gmx-account but if I try to send them through the protonmail-account I get the following error-message:

gnutls.el: (err=[-15] An unexpected TLS packet was received.) boot: (:priority NORMAL:%DUMBFW :hostname 127.0.0.1 :loglevel 0 :min-prime-bits nil :trustfiles (/etc/ssl/certs/ca-certificates.crt /etc/ssl/cert.pem) :crlfiles nil :keylist nil :verify-flags nil :verify-error nil :callbacks nil)
Invalid face attribute :inherit mu4e-contact-face [2 times]
gnutls-negotiate: GnuTLS error: #<process smtpmail>, -15Invalid face attribute :inherit mu4e-contact-face

My protonmail-mbsyncrc-configuration reads

IMAPAccount proton
Host 127.0.0.1
Port 1143
User ****
PassCmd "gpg -q --for-your-eyes-only --no-tty -d ~/.authinfo.gpg | awk '/machine 127.0.0.1/ {print $NF}'"
AuthMechs LOGIN
SSLType STARTTLS
SSLVersion TLSv1.2
CertificateFile ~/.config/protonmail/bridge/cert.pem    

MaildirStore proton-local
Path ~/mail/proton/
Inbox ~/mail/proton/Inbox
SubFolders Verbatim

IMAPStore proton-remote
Account proton

Channel proton
Far :proton-remote:
Near :proton-local:
Patterns "INBOX" "Archive" "Trash" "Spam" "Drafts" "Sent"
CopyArrivalDate yes
Create Both
Expunge Both
SyncState *

my protonmail-mu4easy-context reads

(mu4easy-context
                      :c-name    "proton"
                      :maildir   "proton"
                      :mail      "****"
                      :smtp      "127.0.0.1"
                      :smtp-type ssl
                      :smtp-port 1025)

I've checked that the ports in the mbsyncrc-configuration are the same as in the protonmail-bridge. Any ideas for how to cure this error?