Error

This coin is also thinner than normal, correct? As long as there are no gouges or tooling marks around the rim and unstruck side, then you have a post-mint split planchet error.

This error is caused by a weakness in the metal stock used to strike the coin. If only a portion of the surface splits off, it is called a lamination or delamination. Metal stock with a profound internal weakness can literally cause the coin to split in half and that is what you have there. Sometimes, banging around with other coins and getting knocked around a bit after the coin leaves the mint is enough to cause the split. Splits can also have some "assistance" but that will usually leave traces of tooling marks and you cannot split a coin that does not have an internal weakness.

As for value, I have no idea as I am not familiar with the Croatian error market. I can tell you that it definitely has value as an error.

to me it looks like a split planchet after strike and then brockage, as the obverse image is stamped into the reverse.

This appears to have started out as a split-before-strike planchet that was then struck against another, presumably normal planchet. Value of U.S. cent equivalents are in the $25 - $35 range. The blurred, incuse image on the "blank" face is not a brockage. It is a ghost image of the die-struck design on the opposite face. It's a normal component of the error I've described.