I can't believe this came from a team member. "%20" is the URL encoded representation of a space, not some weird escape sequence.
The correct encondig of the URL above would be "davs://foo.bar/servlet/webdav.infostore/Userstore/Us%C3%89%C3%89%C3%89er%20Na%C3%89%C3%89%C3%89me"
However, I found out that Enpass does neither require nor support pre-encoded URLs, not even "%20", which means you have to put "davs://foo.bar/servlet/webdav.infostore/Userstore/UsÉÉÉer NaÉÉÉme" to make it work.
Edit: This worked for a while but is broken the latest releases. Currently there does not seem to be a way to get spaces or non-ascii characters to work at all. Support says they are working on it, but is has been three months already.