I have now identified what caused my problem. I can easily work around
the issue as it relates to some preamble code that I do not need.
However, the issue points to strange side effects.
I was looking at child documents of a master document. The preamble of
the master document contains the line
\def\Master {}
each of the preambles of the child documents contains the lines
\ifx \Master\undefined
\input{../Preambles/Preamble_Standard}
\input{../Preambles/Preamble_tikz}
\fi
In both LyX 2.0.0 and LyX 2.0.1 the "view master document" command
produces a correct pdf file. However, instant preview does not work for
LyX equations. If I delete the lines "\ifx ... \fi" in the preamble of a
child document and then save and reload the child document, then instant
preview for LyX equations works again in this child document. For LyX
2.0.0 it does not matter whether these lines are present or not. In both
LyX 2.0.0 and LyX 2.0.1 instant preview works for included graphics
whether these lines are present or not.
