Here is a futher point which works right now in this beta-version:
It relates in turn to the point spell check.
First I select the mode »Case Sensitive«. In the last beta 0.5.902 was a bug.
If I'm correct, for example, a lowercase word, after this I could not correct a uppercase word. Sigil has the word that I had written in uppercase, immediately converted to a lowercase.
Earlier, I corrected with the new version 2 eBooks and everything has gone.
By the way ... my computer is running Windows 7/64 bit and I use the 64 bit version of Sigil.