Comment 15 for bug 1664377

This problem is reproducible by installing snapd from 14.04 on Xenial or higher. This happened on your system because you have trusty-proposed enabled in sources.list on a Xenial system. When an SRU of snap is released to trusty-proposed, its version is higher than snapd in xenial-updates and it's pulled on next system update. It is not recommended to enable proposed by default and install all of it unless you really know that it is what you want.

You can fix this situation by removing proposed from /etc/apt/sources.list and installing snapd corresponding to your release. In this case xenial-updates.

I'm closing this report because it is not a supported configuration. Don't hesitate to submit any bug you may find.