Sure. It's the pop-up window. I copied the save file into the current version and the spook was over. It looks like I created this weird bug with a typo in a unit image name. Sorry for unnecessary troubles!
Further notes on the new Custom items Editor:
Manually naming an item is still not a good idea. The names are not saved after closing and restarting the game. If the entry in the text box is not confirmed with >Enter<, other item names can easily be overwritten by mistake with open entries using the scroll arrows. The Custom items screen can only be closed with >Ok< once all entries have been confirmed with the >Enter< key.
Further notes on the new Custom items Editor:
Manually naming an item is still not a good idea. The names are not saved after closing and restarting the game. If the entry in the text box is not confirmed with >Enter<, other item names can easily be overwritten by mistake with open entries using the scroll arrows. The Custom items screen can only be closed with >Ok< once all entries have been confirmed with the >Enter< key.