I was testing today the SVN version of LyX 1.6.0 and 1.5.7. Due to a change in the way the double quotation mark (“) is handled, adding it to Hebrew text resulted in the following LaTeX error:
LaTeX Error: Command \textquotedbl unavailable in encoding HE8
This could be solved by adding the following line to your LaTeX preamble:
This line will define the
\textquotedb to be the double quotation mark, and the document will compile look the same as it looked in older versions of LyX.