Just FYI, ä
This may be fixed in newer versions, but I then maybe it's been well hidden.
I had an test epub that was made with old, much simpler version (0.4...) of Sigil. When I opened it in 0.5.907, there was no html file. (Other books were fine.) So I changed the name of the not showing html, deleted the part containing 'ä' - and now Sigil found it. Put ä back in Sigil, now there's no problems.
Just thought that I should mention it, as most people don't use ü & ö and this could go unnoticed for quite a while.