I solved it ...
Besides deleting the Tomcat cache files
rm -r /var/cache/tomcat7/Catalina/localhost/ciphermail
rm -r /var/cache/tomcat7/Catalina/localhost/web
I also needed to delete the following files:
rm -r /var/lib/tomcat7/webapps/ciphermail
rm -r /var/lib/tomcat7/webapps/web
Then start Tomcat again.
Afterwards I was able to login again ...