The Unknown String error message thingy shows up no matter, I have seen it in Mods that have tons of hand typed everything to ones that folks have tempted fated and used Paradox's Editor...
I believe it is a reading error of the exe file itself. Events or actions happen, like naval combat is where it shows up the most, and the game has nothing really report as to why ship a sank, so unknown string pops up. Just a guess...
Also seen it a lot when folks mod the tech files...