LyX ⇒ Encoding Problem (Export Error)

[auvajs]

Another beginner question and again I have to say I searched the net thoroughly and couldn't find an answer so this is why I'm asking you here.

Document I'm working on:
Language: Czech
Encoding: Unicode (utf8) - chosen in Document settings->Language

I get following Export Error:
Could not find LaTeX command for character '̈́̈́' (code point 0x344)

Description:
Some characters of your document are probably not representable in the chosen encoding.
Changing the document encoding to utf8 could help.

Now what I did was I googled and found that the given character is "COMBINING GREEK DIALYTIKA TONOS". I searched the document but couldn't find such character in it (!!) so I can't remove it though I don't think there is such a character anyway.

I tried changing encoding in Document settings:
Unicode (CJK) (utf8)
Unicode (XeTeX) (utf8)
Unicode (ucs-extended) (utf8x)

but none of them worked too