Thanks for reporting this issue to us. Packages ttyd, libwebsockets-full/openssl comes from OpenWrt packages feed. I have been able to reproduce this issue and created an issue:
This is a problem on all OpenWrt routers, which has any higher or equal version to 18.06, which would like to have installed mosquitto-ssl and ttyd at the same time. It should be fixed there. If it is fixed there, it will be applied to Turris OS in the next update.