I finally had found the problem, better say several problems. First, fpjson unit is half-working, method TJsonObject.Find returns False even if element with the specified name is 100% available.
Next, Lazarus RC2 randomly does not recompile modified units, and no chance to make them compile but manually delete of units folder.
Lazarus itself hang or produces AV periodically
Still some strange problems with unicode - -Mdelphi make it compile (but with non-unicode strings), while -Mdelphiunicode returns strange errors cannot be explained.
Btw, I had commented some components used, as FPC produces internal error compiling them.
Thanks for the advice, will check links